
Class 

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ILLUSTRATED 



COMMON SCHOOL ASTRONOMY. 



WITH 




XPLANATORY NOTES, 



AND 



ClUESTIONS FOR EXAMINATION 



BY JOHN BROCKLESBY, A.M. 

PROFESSOR OF MATHEMATICS AND NATURAL PHILOSOPHY IN TRINITY COLLEGE, HARTFORD, AND AUTHOR 

OF " THE ELEMENTS OF METEOROLOGY ;" " VIEWS OF THE MICROSCOPIC WORLD ;" APC 

" THE ELEMENTS OF ASTRONOMY.' 1 



" The heavens declare the glory of GOD, and the firmament showeth His handywork.' 



INEW-YORK : 
PUBLISHED BY FARMER. BRACE .V CO 

NO 4 CORTLANDT STREET. 
1857. 



Entered, according to Act of Congress, in the year 1857, 

By JOHN BROCKLESBY, 

in the Clerk's Office of the District Court of Connecticut. 



WILLIAM DENYSE, C . A. ALVORD, 

Stereotypy A- Electmtyper, Printer, 

183 William-st M N.Y . Vandewater-st. 






PEEFACE 



Of the multitudes that crowd our Common Schools, it is not 
to be expected that many will devote themselves to the science 
of Astronomy. By far the greater number will be immersed in 
the active concerns of life : and yet, perhaps, all of this numerous 
class would willingly add to the stores of general knowledge 
they are amassing, the great facts and principles of Astronomy, 
if they enjoyed opportunities and facilities for so doing. 

For the aid of such, this little work has been prepared. It 
simply aims to render familiar the important truths and facts 
of the science, avoiding cumbrous details. 

It may be used alone, or as an introduction to some more 
extended work on the subject, such as the " Elements of 
Astronomy." 

Hartford, May 4^, 1857. 



INDEX. 



INTRODUCTORY CHAPTER. 

Astronomy defined, .page 9 

Solar System, 10 

Mode of conducting Astronomical Investigations, 11 

EXPLANATORY CHAPTER. 

Angle, 14 

Right Angle, 15 

Triangles, 16 

Similar Triangles, 16 

A Plane Surface, 17 

A Plane Figure, 17 

Ellipse, 17 

To construct an Ellipse, 18 

Eccentricity, 19 

A Sphere, 19 

Poles of a Circle of a Sphere, 20 



PAET FIEST. 

THE EARTH IN ITS RELATION TO OTHER HEAVENLY BODIES. 

CHAPTER I. 

ITS FORM, SIZE, AND ROTATION. 

The Form of the Earth, „ 22 

Size, 24 

Rotation, 25 

CHAPTER II. 

THE HORIZON. 

Sensible Horizon, 26 

Rational Horizon, 27 

Plane of the Horizon not fixed in Space, » 27 



iv INDEX 



Zenith and Nadir, page 28 

Changing Aspect of the Heavens, arising from the Kotation of the 

Earth, 28 

Why the Stars appear to describe Circles, 30 

Pole of the Heavens, and Pole Star, 31 

Changing Aspect of the Heavens, arising from Change in Latitude,. . 31 

Circle of Perpetual Apparition, 33 

Circle of Perpetual Occultation, 34 

Aspect of the Heavens at the Equator, 37 

At North Latitude Forty Degrees, 37 

At the North Pole, * 38 

Latitude of any Place equal to the Elevation of the Pole of the Heavens, 88 . 



CHAPTEE III. 

OF THE MODE OF DETERMINING THE PLACE OF A HEAVENLY BODY. 

Celestial Sphere, Poles, Axes, and Meridians, 40 

Equators, 42 

Vertical Circles, , 42 

The Position of a Star — how determined, 42 

Azimuth and Altitude of a Star, 43 

Declination and Eight Ascension, 44 

Ecliptic, 45 

The Signs, . 46 

Zodiac, 46 

CHAPTEE IV. 

OF THE MEASUREMENT OF TIME. 

Of the Time occupied by the Earth in performing one Rotation — how 

determined, 47 

Standard Unit of Time, 49 

Of the Sidereal and Sola?' Day, 50 

Apparent Time, 52 

Mean Solar Time, . 52 

Astronomical Time, 52 

The Tropical Year, 53 

Egyptians, 54 

Mexicans, 54 

CHAPTER V. 

OF THE EARTH'S ORBIT, AND THE SEASONS. 

The Earth's Orbit, 56 

The Seasons, 57 

Spring, 59 



INDEX 



Summer, page 59 

Autumn, 62 

Winter, 62 

Polar Winters, 64= 



PART SECOND. 

SOLAR SYSTEM. 

CHAPTER I. 

THE SUN. 

Diameter, 66 

Size or Bulk, 67 

Quantity of Matter, 68 

Weight of Bodies at the Surface of the Snn, 68 

Solar Spots, 70 

Size and Number, 71 

Motion of the Spots, 72 

Rotation of the Sun on its Axis, 75 

Physical Nature of the Sun, 75 

Temperature at the Sun's Surface, 77 

CHAPTER II. 

THE MOON. 

Distance — Diameter, 78 

Moon's Phases, 79 

From New Moon to the First Quarter, 79 

From the First Quarter to Full Moon, 80 

From Fall Moon to the Third Quarter, 81 

From the Third Quarter to New Moon, 81 

What the Phases prove, 84 

Sidereal Month, 84 

Synodical or Lunar Month, 85 

Physical Aspects of the Moon, 85 

Lunar Mountains, 87 

Lunar Craters, 88 . 

Lunar Volcanoes,. 91 

Bulk— Mass— Density, 92 

The Moon's Orbit, 92 

The Line of the Nodes, 92 

The Moon always turns the same Face towards the Earth, 93 

Length of the Lunar Day, 95 

The Appearance of the Earth, as seen from the Moon, 95 



vi INDEX. 



CHAPTER III. 

ECLIPSES OF THE SUN AND MOON. 

Lunar Eclipses, page 97 

Of the Earth's Shadow, 98 

Red Light of the Disk, 100 

Earliest Observations of Lunar Eclipses, 101 

Eclipses of the Sun, 101 

Form of the Eclipse, 102 

Shadow of the Moon, 103 

Total Eclipse of the Sun, , 104 

Solar and Lunar Eclipses — Points of Difference, 108 

Frequency of Eclipses, 108 

CHAPTER V. 

THE PLANETS. 

Universal Attraction, 110 

Kepler's Laws, Ill 

The Planets and their Solar Distances, 112 

Apparent Size, 118 

Kepler's Law of Distances, 113 

Division of the Planets, • 114 

Inferior Planets, 114 

Mercury, 116 

Rotation on its Axis, 117 

Phases, 118 

Transit of Mercury, 118 

Mass and Density, 119 

Venus, 119 

Rotation — Phases, 120 

Splendor of Venus, 122 

Transit of Venus, 122 

Mass and Density, 122 

The Earth, 123 

Superior Planets, 1 23 

Mars, 123 

Splendor — Physical Aspect, 124 

The Asteroids, 126 

Table of Asteroids, 128 

Jupiter, 127 

Physical Aspect of Jupiter — Belts, 129 

Satellites of Jupiter—Their Discovery, 130 

Their Magnitudes — Diameters — Distances — Periods of Revolution, 131 



INDEX. vii 



Kepler's Laws — Eotation, page 131 

Transits and Eclipses of the Satellites, 132 

Saturn, 132 

Physical Aspect, 1 33 

Eing of Saturn — its Discovery, 133 

Form and Constitution, 133 

Eotation and Position, 134 

Divisions of the Eing, 134 

Dimensions of the Eings,. 136 

Satellites of Saturn, 137 

Hyperion, 137 

Uranus or Herschel, 137 

Aspect — Diameter — Mass,. 138 

Rotation, 138 

Distance — Periodic Time, 138 

Satellites of Uranus, 138 

Neptune — History of its Discovery, 139 

Name — Diameter— Mass — Density, 140 

Orbit — Distance — Periodic Time, 140 

Has Neptune a Eing ? , 141 

The Satellite of Neptune, 141 



CHAPTER VI 



COMETS. 



Constitution, 141 

Number of Comets, 142 

Splendor and Size, 142 

Comet of 1680, 143 

Comet of 1843, 144 

Orbits, 144 

The Comets of Haliey, Encke, Biela, and Faye, 145 

Nature of Comets, 145 

CHAPTER VIL 

TIDES. 

Tides denned, 146 

Cause of the Tides, 148 

Solar Influence, 148 

Spring and Neap Tides, 149 

Actual Heights of the Tide, 151 

No Tides except on the Ocean, and on Seas connected with it, 152 



Tiii INDEX. 



PART THIRD. 

THE STARRY HEAVENS. 
CHAPTER I. 

ON THE FIXED STARS IN GENERAL, AND THE CONSTELLATIONS. 

The Fixed Stars, page 153 

Magnitudes, 153 

Number of Stars, 154 

The Constellations — Their Use, , 155 

Principal Constellations, , 156 

Distance of the Fixed Stars, 157 

Nature and intrinsic Splendor of the Fixed Stars, 158 

CHAPTER II. 

DIFFERENT KINDS OF STARS — STELLAR MOTIONS — BINARY SYSTEMS. 

Periodical Stars, 158 

Mira, 158 

Algol, 159 

Temporary Stars, 159 

Double Stars, 161 

Castor — Alpha Centauri, 161 

Colored Double Stars, 161 

Triple and Quadruple or Multiple Stars, 162 

Binary Stars, 162 

Orbits — Periodic Times, 163 

CHAPTER III. 

STARRY CLUSTERS — NEBULAE — NEBULOUS STARS — ZODIACAL LIGHT — MAGELLAN 
CLOUDS — STRUCTURE OF THE HEAVENS. 

Starry Clusters, 164 

Milky Way or Galaxy, 164 

Nebulae— Their Constitution, 166, 168 

Number and Distance of Stellar Clusters and Nebulie, 168 

Their Physical Structure, 169 

Nebulous Stars, 160 

Zodiacal Light, 169 

Aspects — Size— Nature, 170 

Structure of the Heavens, ^ 171 

Ptolemaic, Tychonic, and Copernican Systems, 171 



ASTRONOMY. 



INTRODUCTORY CHAPTER. 

1. Astronomy is that branch of Natural Science 
which treats of the magnitudes, distances, constitu- 
tions, and motions of the Heavenly Bodies, and the 
laws which regulate them. 

2. The Heavenly Bodies consist of moons, planets, 1 
comets, 2 and suns ; and possibly a fifth class exist 
called nebulae?- To moons, planets, and suns, the 
general name of stars is often given. 

3. The mode of union existing among the heavenly 
bodies is the following : One or more moons revolve 
aoout a planet ; several planets with their attendant 
moons revolve about a sun, around which, likewise, 
sweeps a numerous train of comets. A sun with its 
assemblage of planets and comets constitutes a system. 

1. Planet, from the Greek word planetes, signifying a wanderer: a 
star that changes its place in the heavens. 

2. Comet, from the Latin word coma, a head of hair, this body pre- 
senting a hairy appearance. 

3. The name of nebulce is given by astronomers to certain objects in 
the distant heavens which appear like small clouds, or specks of mist. 
True nebulae are supposed to be vast collections of unformed matter, 
thinly diffused through space. Nebulae is a Latin word, signifying 
mists, or clouds. 

What is Astronomy ? What do the heavenly bodies consist of % What is the 
mode of union between heavenly bodies ? 

1* 



10 



ASTRONOMY. 



4. The investigations of astronoiners tend to prove 
that these systems are not fixed in space, but revolve 
like planets about some common central point, or 
body. And we have reason for believing that this 
mode of arrangement extends throughout all space, 
groups of systems rising one above the other in mag- 
nitude ; the lesser circling around the greater, until 
at length their vast aggregate embraces and com- 
pletes the Universe. 

5. Solab System. The sun with his train of planets, 
moons, and comets, forms the Solar System. 

6. The number of planets already known is fifty* 
Thirty-nine of these have been discovered within the 
last twelve years, and others will doubtless be detected. 
The names of the planets are given in the following 
table, in the order of their distances from the sun, 
beginning with the nearest. 

TABLE OF THE PLANETS. 



MERCURY, (nearest), EARTH, VENUS, MARS. 

The Asteroids, between Maes and Jupiter. 



FLORA, 


PHOCEA, 


POMONA, 


CALLIOPE, 


HARMONIA, 


MASS ALIA, 


PROSERPINE, 


PYSCHE, 


ISIS, 


HEBE, 


FIDES, 


THEMIS, 


MELPOMENE, 


LUTETIA, 


EUNOMIA, 


HYGEIA, 


CLIO, 


FORTUNA. 


THALIA, 


EUPHROSYNE, 


EUTERPE, 


PARTHENOPE, 


JUNO, 




URANIA, 


THETIS, 


CIRCE, 




VESTA, 


AMPHITRITE, 


CERES, 


LEUCOTHEA, 


POLYMNIA, 


EGERIA, 


LjETITIA, 


ATALANTA, 


METIS, 


ASTREA, 


PALLAS, 


LEDA, 


IRIS, 


IRENE, 


BELLONA, 


DAPHNE. 


JUPITER, SATURN 


, HERSCHEL, or 


URANUS, NEPTUNE (most distant). 



What is a system ? What is the solar system 1 How many planets are now 
known? How many have been discovered within the last twelve years ? Give the 
names of the planets. 



ASTRONOMY. H 



7. All the planets between Mars and Jupiter are 
termed Asteroids. 1 In the annexed cut, Fig. 1, a 
view of the solar system is presented. The Roman 
numeral, I, represents the orbit 2 of Mercury ; II, that 
of Venus ; III, that of the Earth ; IY, that of Mars ; 
V, the orbit of the nearest asteroid ; VI, that of the 
most remote asteroid ; VII, is the orbit of Jupiter ; 
VIII, of Saturn ; IX, of Herschel ; and X, of Nep- 
tune. In the cut the distances of the several planets 
from the sun bear the same relation to each other as 
their actual distances. 

8. The Mode of Conducting Astronomical Inves- 
tigations. "When an artisan wishes to ascertain the 
dimensions of a stick of timber, he does so by means 
of a rule, the length of which he knows, and thus he 
obtains the solidity of the log in feet and inches. 

When, likewise, we wish to determine the speed of 
a locomotive, we measure by the aid of a w-atch the 
time taken to pass over a known number of miles. 
Thus unknown magnitudes and motions are ascer- 
tained by comparing them with such as are known. 

9. In astronomical investigations we pursue a like 
course, and begin w T ith determining the size, motions, 
and form of the Eajrth, with other important partic- 
ulars that are within our reach. We thus obtain 
fixed standards of measurement, whereby we are ena- 

1. Asteroids. From two Greek words : aster, a star, and eidos, like. 
Like a star, because all these planets are very small. 

2. Orbit means the path of a planet about the sun. So called from 
the Latin word, orbis, a circle, a circuit. 

What are the Asteroids, and where situated? Explain the figure. In what 
manner are astronomical investigations conducted? 



12 ASTRONOMY. 



bled to push our inquiries beyond the earth, and to 
compute the distances, times, motions, and velocities 
of many of the bright orbs 3 that glitter about us, and 
the extent of the vast spaces through which they 
move. In the study of Astronomy our attention is, 
therefore, directed First to the Earth in its relation 
to the rest of the heavenly bodies. Secondly ', to the 
Solar System. Thirdly, to the Starry Heavens, of 
which this system is a part. . 

3. The stars are frequently called orbs, from their round figures. 
(Latin,) a circle. 



ASTRONOMY 



18 



FIG. 1. 




SOLAR SYSTEM. 



14 ASTRONOMY. 



EXPLANATOKY CHAPTER. 

10. In learning Astronomy it is necessary for the 
pupil at the outset to know the meaning of certain 
mathematical and philosophical terms and expressions 
which are constantly occurring - in the discussion of 
astronomical subjects. These must "be mastered in 
order to obtain a clear understanding of the science, 
and yet they are by no means difficult to comprehend. 
The most important of these are explained in the pre- 
sent chapter. The meaning of other terms and 
phrases will be given as they occur in the book. 

11. Angle. An angle is the opening or inclination 
between two lines that meet each other ; thus, in Fig. 

FIG. 2. 




AN ANGLE. 



2, the line AB meets the line BC, and the opening 
between them is called the angle B, or the angle 
ABC ; the letter at the point of meeting always 
being placed in the middle. 

The size of an angle is computed as follows. The 

In learning astronomy, what is it necessary for the pupil to do at the outset 1 
What ii an angle ? How is its size computed? Describe from the figure. 



ASTRONOMY. 15 



circumference of any circle being divided into 360 
equal parts, each part is called a degree / a degree 
being divided into 60 equal parts, each part is called 
a minute * a minute being divided into 60 equal 
parts, each part is called a second. If, now, we take 
the point B, as the centre of a circle, and describe 
the circumference DEF, cutting the two lines, AB 
and CB in any two points, as E and F, the number of 
degrees, minutes, and seconds contained in the part of 
the circumference, EF, included between the two 
lines, AB and CB, gives the value of the angle, ABC. 
For example, if the length of the circumference, 
DEF, was 360 inches, and the part, EF, contained 40 
inches and nine-sixtieths (40^-) of an inch, ABC 
would be an angle of forty degrees and nine minutes 
(40° 9'.) Degrees are denoted by the following char- 
acter, ° ; minutes thus, ' ; seconds thus, ". 

12. A Right Angle. A right angle contains 90°, 
and can be thus constructed. Draw two diameters 
through any circle, dividing the circumference into 

FIG. 3. 




RIGHT ANGLE. 



How are degrees, minutes, and seconds denoted? What is a right angle, and 
how it it constructed ? 



16 ASTRONOMY, 



four equal parts, and each of the angles at the centre 
will be a right angle, for since the whole circumfer- 
ence contains three hundred and sixty degrees, one- 
fourth of it contains ninety degrees. Thus, in Fig. 3, 
the two diameters, AB and DE, dividing the circum- 
ference of the circle, A, D, B, E, into four equal 
parts, make each of the angles at the centre, right 
angles, viz., ACD, DCB, BCE, andECA. 

13. Triangles. A triangle is a figure that is bounded 
by three lines, either curved or straight, and contains 
three angles : hence its name, derived in part from a 
Latin word, tres, meaning three. The sum of the 

FIG. 4. FIG. 5. 





A RECTILINEAR TRIANGLE. A RECTILINEAR RIGHT-ANGLED TRIANGLE. 

three angles of any rectilinear 1 triangle is always 
equal to 180°. Fig. 4 represents any such triangle, 
and the sum of its angles is equal to 180°. A right- 
angled rectilinear triangle is one that contains one 
right angle. Thus, Fig. 5 is a right-angled triangle, 
since it contains a right angle, viz., ABO. 

14. Similar Triangles. Similar triangles are those 
which have all the angles of one triangle equal to 

1. Rectilinear, from rectus, straight, and linea, a line, (Latin,) 

STRAIGHT-LINED. 

What is a triangle ? How many degrees does it contain ? Refer to figure. What 
is a right-angled triangle? Refer to figure. What are similar triangles? Explain 
from figure. 



ASTRONOMY. 17 



those of the other, each to each, and the sides forming 
the equal angles proportional ; thus, in Figs. 6 and 7, 
the triangles, A^C 1 and ABC, are similar, because 

FIG. 6. FIG. 7. 





SIMILAR TRIANGLES. 



the angle B 1 equals B, A 1 equals A, C 1 equals C, and 
the side A'B 1 : AB : : A l C l : AC\ and so of the 
other sides. 

15. A Plane Surface. A plane surface is such that 
if any two points in the surface are connected by a 
straight line, every part of that straight line touches 
the surface. To illustrate : The surface of tranquil 
water in a pond is a plane surface, because if any two 
points on the surface are taken, and are connected by 
a perfectly straight rod, every part of the lower side 
of the rod touches the water. Such a surface is some- 
times termed a plane. Thus, the surface of this page, 
when pressed flat, is a plane surface, or plane. 

16. A Plane Figure. A plane figure is one whose 
bounding line or lines are situated in the same plane. 
The flat cover of this book is a plane figure. 

17. Ellipse. An ellipse is a plane figure, bounded 
by a curved line, and so constructed that if two 
straight lines are drawn from two points within, 
called the foci, to any point in the curve, the sum of 

What is a plane surface ? What a plane figure ? What is an ellipse ? De- 
scribe it from figure. 



18 ASTRONOMY. 



these lines is invariably the same for the same ellipse. 
Thus, the annexed figure is an ellipse. F and F 1 , the 
foci, and if straight lines are drawn to any points, as 

FIG. 8. 




AN ELLIPSE. 



M and M 1 ; FM added to F X M equals FM 1 added to 
F l M*, and so of lines drawn to any other point. The 
line DD 1 , drawn through the foci and terminated by 
the curve, is called the major axis of the ellipse. The 
line PP 1 , drawn through C, the middle of DD 1 , or 
the centre of the figure, is the conjugate axis. 

18. To Construct an Ellipse. Stick two pins into 
a piece of paper, at a short distance from each other, 
as at F and F 3 , and pass over them a loop of thread, 
place a pencil in the loop, and keeping the thread 
tight, a triangle will be formed like FMF 1 , the pencil 
being at M. Passing the pencil completely round F 
and F 1 , its point will mark out an ellipse. For since 
in making the circuit, the length of the loop does not 
change, neither the distance between F and F 1 , it ne- 
cessarily follows that the sum of the distances from 
the pins to the pencil: viz., F 1 ]^, FM, &c, is in- 
variable. 

Construct an ellipse. 



ASTRONOMY. 19 



19. Eccentricity. Ellipses differ among themselves. 
If the foci are near the centre of the ellipse, the 
ellipse approaches the form of a circle ; but if the 
foci depart widely from it the length of the conju- 
gate axis is small in proportion to that of the major 
axis, and the ellipse is said to be very eccentric. 1 

The distance from the centre to either focus, viz., 
FC, or F X C, is termed the eccentricity of the ellipse. 

FIG. 9. FIG. 10. 




In Figs. 9 and 10, two ellipses are exhibited which 
differ greatly in their eccentricity — one being almost 
a circle, and the other very oval. 

20. A Sphere. A sphere is a solid, bounded by a 
curved surface, every point of which is equally dis- 
tant from a point within^ called the centre / every 
line passing through this centre, and limited by the 
surface, is a diameter. The half of this line is a ra- 
dius of the sphere. Thus, in Fig. 11, representing a 

1. Eccentric, from ex, out of and centrum, centre, (Latin) out of the 
centre. 



What is meant by the term eccentricity 1 

What is a sphere ? Describe it from the figure with its lines and sections. 



20 



ASTRONOMY 



FIG. 11 




A SPHERE WITH iTS SECTIONS. 



sphere, the points D, O, L, A, E, H, N, &c., are all 
equally distant from the centre C. DE and AB are 
diameters, and CP, CL, CA, CB, &c, are each a ra- 
dius. 

If a plane passes through a sphere, any section it 
makes with the sphere is a circle. A great circle 
passes through the centre of the sphere, all other 
circles are small circles. Thus, in the figure, AFB is 
a great circle, and LHN and OP small circles. 

21. Poles of a Circle of a Sphere. The poles of a 
circle of a sphere are points on the surface of a 
sphere, equally distant from every point in the cir- 
cumference of that circle. Thus, D is the pole of the 
circle LHN, because the curved lines DH, DN, and 

What are the poles of a circle of a sphere ? Explain from figure. 



ASTRONOMY. 21 



DOL, and all others drawn to the circumference 
LHN, are equal to one another. For the same reason 
D is the pole of the circles OP and AB. It will also 
be seen that the point E is situated like D, with re- 
spect to these circles, since the curved lines EBN and 
EAL are equal, as likewise ELO and EOT\ Each 
circle of a sphere has therefore two poles. 

In a great circle the poles are each ninety degrees 
distant from the circumference of the circle ; thus, in 
the great circle AB, the poles E and D are each 
ninety degrees from its circumference AFB. 



PART FIRST. 



THE EARTH IN RELATION TO OTHER HEAVENLY 

BODIES. 



CHAPTEE I. 

ITS FORM, SIZE, AND ROTATION. 

22. Its Form. The earth appears to our view to 
be nothing more than a vast broken plain, rising into 
mountains, sinking into valleys, and spreading out 
into lakes, seas, and oceans ; but a careful investiga- 
tion removes this erroneous impression, and proves, 
First, that the general surface of the earth is eurved ; 
Secondly r , that the mass itself is nearly spherical in 
form ; Thirdly r , that it rests upon nothing. 

23. The facts are established by several independ- 
ent proofs. In the first place, when a vessel sails 
from the shore, the spectators upon the strand, as they 
watch her lessening in the distance, at length per- 
ceive the hull gradually sinking below the line of the 
horizon ; x next the lower sails disappear, and the last 
objects that are seen are the tops of the masts, on a 
level with the* distant waters ; and this is the case in 

1. Horizon, a boundary. It here means the line that apparently 
divides the surface of the earth from the sky and limits our view. Its 
full meaning is explained in Arts. 27, 28, 29. 

What does Part First treat of? What does Chapter First treat of? What 
appearance does the earth present ? What facts have been proved by careful in- 
vestigation ? State the several proofs of these facts. 



ASTRONOMY. 23 



whatever direction the vessel sails. Secondly, navi- 
gators have repeatedly sailed around the earth, by 
advancing constantly in the same direction ; arriving 
at length at the port from whence they departed. 

Thirdly. The horizon is circular, which would not 
be the case unless the earth was spherical. 

Fourthly. When the sun, earth, and moon are so 
situated that they are all in the same straight line, 
the earth being in the middle, the latter casts a 
shadow upon the moon, causing an eclipse. This 
shadow is seen to be 'circular in form, thus proving 
that the earth is round. 

Fifthly. Since the sun, and the nearest heavenly 
bodies are seen to be round, we naturally infer that 
the earth does not constitute an exception, but has 
also a similar form. 

Sixthly. From observations and actual measure- 
ments, mathematicians are able to compute the dis- 
tances of places on the earth's surface from its centre ; 
in numerous places widely differing in latitude and 
longitude ; these distances have teen computed, and 
are found to be in all instances nearly equal. This 
fact proves the spherical shape of the earth, since a 
sphere alone of all solid bodies possesses the property, 
that the distance from the centre to any point on its 
Burface is everywhere the same. 

In view of all these facts we conclude that the 
earth is a body having a curved surface that is nearly 
spherical in shape, and that it rests upon nothing. 

24. We say nearly spherical, for according to the 
best calculations of astronomers, the earth swells out 
at the equator, the diameter which passes through 



24 ASTRONOMY 



the centre and the poles, 1 being about one three hun- 
dredth part (3^0 th) shorter than any diameter that 
passes through the equator. To such a solid geome- 
tricians have given the name of oblate spheroid. 

25. Size of the Earth. The length of the polar 
diameter of the earth is 7,899 miles, and that of the 
equatorial diameter 7,925^- ; the latter being longer 
than the former by 26^ miles. Thus in Fig. 12, if PS 
represents the polar diameter, and EQ the equatorial, 
EQ is longer than PS by 26£ miles. 

The distance around the earth, that is, its circum- 
ference, is about 25000 miles. 

FIG. 12. 






Q 



s 

THE EARTH. 



1. The polar diameter of the earth is the imaginary line or axis about 
which the earth rotates, like a wheel on an axle. Its extremities are 
the poles of the earth. A diameter drawn at right angles, to the polar 
diameter, and passing through the centre of the earth, is an equatorial 
diameter. See Fig. 12, where P and S are the poles of the earth, and 
PS the polar diameter. 

Is the earth exactly spherical ? How much shorter than the equatorial diameter 
is the polar ? What is the geometrical name of a solid body shaped like the earth ? 
What is the length of the polar diameter of the earth? What of the equato- 
rial? What is the distance around the earth? 



ASTRONOMY. 25 



26. Rotation of the Earth. To every one who can 
see, it is one of the most familiar sights in nature to 
behold the sun ascend the eastern sky, attain its noon- 
tide splendor, and at length set beneath the western 
horizon. And when night approaches, the stars ap- 
pear moving in the same order ; rising successively 
above the eastern horizon, and passing over to the 
west. This motion of the celestial bodies can be 
explained in two ways, either by supposing that the 
earth is at rest, and all the luminaries of the sky 
actually revolve about it, or that their motion is only 
apparent 1 , the earth itself really rotating while the 
celestial orbs remain immovable. 

The first theory was received as the truth for ages, 
until the discoveries of scientific men at length 
showed its falsity, and established, by undeniable 
proofs, the fact of the rotation of the earth on its axis. 
The period of rotation, as we shall hereafter see, is 
divided into twenty-four equal parts, called hours. 

1. When a person sails from the shore with a steady wind, the shore 
apparently moves backward, while the ship seems to be stationary, 
though the observer knows all the while that the true state of the case 
is exactly the reverse. The ship moves, but the shore is fixed. 

In how many ways can the motions of the celestial bodies be explained? What 
are those ways? Which one for ages was received as true? Has it been proved 
false ? How is the period of rotation divided 1 



26 ASTRONOMY 



CHAPTEE II. 

THE HORIZON. 

27. Horizon is an astronomical term derived from 
the Greek word orizon, signifying boundary ', and of 
these boundaries there are two. 

28. Sensible Horizon. The first is the sensible or 
visible horizon, of which we have already spoken. It 
is the line apparently separating the earth and sky, 
and which a spectator upon the expanse of ocean, or 
on a vast unbroken plain, perceives to be a circle. 

The plane of the visible horizon is regarded as 
touching the earth at the point where the spectator 
stands, though strictly speaking this is not the case, 
since on acccount of the depression of the visible 
horizon the point where the spectator stands is a little 
above its plane. This is evident by referring to Fig. 
13, where the circle E, represents a section of the 
globe, S the place of the spectator, and the circular 
line VH, a part of his visible horizon. Now it is 
evident at a glance, that owing to the curvature ol 
the earth the plane of the visible horizon, which 
takes the direction Y^HH 1 , is necessarily below the 
parallel plane that touches the earth at S, the place 
of the spectator, and takes the direction of ASA 1 . 
Nevertheless the difference in distance between the 
two planes is usually so small that they are generally 
regarded as coinciding; the plane of the horizon 
being supposed to pass through S. 

What does Chap. II. treat of? What is the meaning of the term horizon ? 
Give the meaning of the term sensible horizon, and explain from Fig. 13. 



ASTRONO M Y 



27 



29. Rational Horizon. The second or rational hor- 
izon is a vast imaginary circle whose plane passes 



FIG. 13. 




HORIZON EXPLAINED, 



through the centre of the earth and reaches to the 
sky, dividing the earth and sky into two hemispheres, 
and is parallel to the plane of the visible horizon. It 
is also represented in direction in Fig. 13, by the 
line EE 1 . 

30. Plane of the Horizon not fixed in Space. We 
have said that the plane of the horizon touches the 
surface of the earth at the point where the spectator 

What is the rational horizon 1 

la the plane of the horizon fixed in space ? 



28 ASTRONOMY. 



stands, or in other words, is at right angles at this 
point to a plumb-line 1 passing through this same 
point. Now the direction of the plumb-line varies 
at every point of the earth's surface. Consequently, 
there are as many horizons, both sensible and ration- 
al, as there are such points ; and the planes of these 
horizons take all possible directions. Thus, in Fig. 
13, if S, S 1 , S 2 , represent the stations of different 
spectators upon the earth's surface, it is clear that the 
planes of the horizons of each, viz, AA 1 , A 2 A 3 , A 4 A 5 , 
cake different directions. 

31. Zenith and Nadir. The point in the heavens, 
in the direction of the plumb-line, exactly over the 
head of an observer, is the zenith ; and the point in 
the heavens beneath him in the opposite direction is 
the nadir. Thus, in Fig. 13, Z is the zenith of the 
place S, and N. its nadir. - 

Since the horizon of an observer changes at every 
step, it necessarily follows that his zenith and nadir 
also change. The zenith of the place directly beneath 
us on the opposite side of the earth is our nadir, and 
its nadir our zenith. In Fig. 13, Z is the zenith at S, 
and N the nadir. At S 1 , Z 1 is the zenith and N 1 the 
nadir. At S 2 , Z 2 is the zenith and N 2 the nadir. 

32. Changing Aspect of the Heavens arising 

1. If a ball of lead is tied to one end of a string, and the other end 
held up so that the ball can swing freely, the direction the string takes 
when the ball is at rest is the direction of the plumb-line. Plumbum 
is the Latin word for lead. 

How many horizons are there ? Explain from figure. 

What is meant by zenith and nadir? Explain the changing aspect of the hea- 
vens arising from the rotation of the e^rth. 



ASTRONOMY. 29 



fkom the Rotation of the Earth. Having learn- 
ed the fact of the rotation of the earth, and of 
the full meaning of the term horizon, we will now 
contemplate the aspect of the starry sky, remember- 
ing all the while that we are not stationary, but stand- 
ing on the surface of a rolling ball. 

33. If, in our latitude, upon a clear evening, we 
take a position upon some commanding eminence, 
we perceive the whole of the overarching sky studded 
with multitudes of glittering stars, down to the very 
line which separates the earth from the heavens. 
Some bright cluster may perhaps be seen just hang- 
ing above the western horizon, while another may 
arrest our attention in the eastern sky. After a short 
time, when we turn our eyes again towards the west- 
ern group it is no longer visible, but has sunk beneath 
the horizon, while the cluster in the east has attained 
a loftier elevation. 

34. By a longer and closer observation we And 
that all the stars have this common motion from east 
to west, and that they appear to move in circular 
paths. 

35. Our knowledge of the rotation of the earth 
renders these appearances perfectly intelligible. The 
stars are not really in motion, but only appear to be, 
for the earth in its rotation from w r est to east is con- 
stantly depressing the eastern part of the horizon and 
elevating the western, so that a star rises in conse- 
quence of the eastern horizon being carried below it, 
and sets because the western horizon is carried up to 
it and above it. 

36. This point is illustrated in Fig. 14, where circle 



so 



ASTRONOMY 



E represents the earth rotating from west to east, as 
shown by the direction of the arrows. If a person is 
at M, on the surface of the earth, the plane of his 
horizon is in the line HH 1 , and the star S is above his 
western horizon, and the star S 1 below his eastern 
horizon. But when the earth in its rotation brings 
the person into the position M 1 , the plane of his hori- 
zon has been so changed as to take the direction 
H 2 H 3 , and the star S has set below the western hori- 
zon, while the star S 1 has risen above the eastern. 

FIG. 14. 



WEST. 




EAST. 



CHANGING HORIZONS. 



37 



AVhy the Stars appear to describe Circles. 
The apparent circular paths of the stars is the result 
of our own circular motion on the surface of the 
earth ; for, not perceiving ourselves to move, these 
orbs appear to have the kind of motion that really 
belongs to us. This illusion is the same as that which 
happens when two trains of cars coming from opposite 



Explain why the stars appear to describe circles. 



ASTRONOMY. 31 



directions stop side by side in a depot, and a passen- 
ger in one looks out at the opposite stationary train, 
the moment his own starts ; unconscious of his own 
motion, the train at his side appears to him to move 
in a contrary direction to that in which he himself is 
actually proceeding. If his own train moves in a 
straight line, the other appears to do so likewise ; but 
if the former moves in a circular track, such is the 
apparent course of the latter. In like manner a spec- 
tator moving in a circle upon the rolling surface of 
the globe, sees the stars moving in circles in a direc- 
tion contrary to his own, imagining himself all the 
while to be at rest. 

38. Poles of the Heavens and Pole Stab. The 
two points in the heavens towards which the axis of 
the earth is directed are the north and south poles of 
the heavens. The north pole of the heavens is 
directly above the north pole of the earth, and the 
south pole of the heavens directly above the south 
pole of the earth. A little way from the north pole 
of the heavens is a bright star called the pole star. 

39. Changing Aspect of the Heavens arising 
from change in Latitude. A person standing on the 
surface of the earth at the equator, has the plane of 
his horizon parallel to the axis of the earth. The 
poles of the heavens are consequently situated in this 
plane, and his horizon appears to pass through them. 
All the circles of daily motion 1 are therefore perpen- 

1. By the term circles of daily motion, is understood the circles de- 
scribed by the heavenly bodies in their apparent daily motion from east 
to west. 

What is meant by the north and south poles of the heavens ? Where is the pole 
star situated 1 Describe the celestial, appearances at the equator. 



32 ASTRONOMY. 



dicular to the horizon. A star which rises in the east 
passes directly overhead and sets in the west, and 
each orb describing half a circle above and half be- 
low the horizon is, therefore, visible for twelve hours, 
and invisible for the same space of time. 

40. If the observer now advances northerly his 
horizon constantly changes in position, being de- 
pressed below the north pole of the heavens, and 
elevated above the south the same number of degrees 
and parts of a degree that correspond to his latitude. 
Thus, if he has arrived at ten degrees, north latitude, 
the northern pole is ten degrees above his horizon, 
and the southern ten degrees below it. If, at fifty 
degrees, thirty minutes, north latitude, the north pole 
is fifty degrees and thirty minutes above the horizon, 
and the southern as much below it. And if it were 
possible for a person to attain the distance of ninety 
degrees, north latitude, and stand upon the northern 
pole of the earth, his horizon would be parallel to the 
equator, the north pole of the heavens would be 
ninety degrees from the horizon, that is, in the zenith, 
and the southern pole of the heavens would coincide 
with his nadir. 

41. This change in the relative positions of the 
poles of the heavens and the horizon, produces a cor- 
responding change both in the inclination of the cir- 
cles of daily motion to the horizon, and in the period 
of visibility of different stars. For, all the stars ap- 
parently revolve in circles, at right angles to the 

What changes oocur as an observer advances towards the north? If he stood 
upon the pole, where would the north pole of the heavens be ? Where the south ? 
What corresponding changes are produced by the variations in position incident to 
the poleB of the heavens and the horizon ? 



ASTRONOMY. 33 



imaginary line joining the poles of the heavens, called 
the axis of the hecwens : and as the north pole of the 
heavens is elevated more and more above the horizon, 
these circles* of daily motion must cut the horizon 
more and more obliquely, until at the north pole of 
the earth a person would see the stars revolving about 
him in circles parallel to the horizon. 

42. Moreover, when the north pole of the heavens 
rises above the horizon, and the south sinks below it, 
it is only those stars that are situated directly above 
the earth's equator which are visible in a clear sky 
for twelve hours above the horizon, and are absent as 
long below it, since the centre of their circle of daily 
motion is alone in the plane of the horizon. All the 
stars to the north of the equator have the centres of 
their circles of daily, motion more and more elevated 
above the plane of the horizon according as they are 
situated farther to the north. The circumferences of 
the circles they describe, it is true, become smaller 
and smaller, but the arcs 1 described above the hori- 
zon are proportionally larger ; and consequently the 
time that a star is visible increases up to a certain 
limit from the equator towards the north. 

43. Circle of Perpetual Apparition. There are 
stars which never set ; for when an orb is at a less 
distance from the pole than the horizon is, it is evi- 

1. Arc, any portion of the circumference of a circle. 

What is the axis of the heavens? 

How would the stars appear to revolve to a spectator at the north pole ? What is 
said respecting the times of visibility of stars at the equator and north of the 
equator ? 

Are there stars which never set ? What is meant by the circle of perpetual ap- 
parition ? 

2* 



34 ASTRONOMY. 



dent that such a star will continue to revolve about 
the poles without ever sinking below the horizon. A 
circle around the elevated pole having a radius equal 
to the altitude of the pole above the horizon is called 
the circle of perpetual apparition, because the stars 
within it never set. This circle changes in size with 
the change of latitude. In latitude ten degrees its 
radius is ten degrees ; in latitude fifty degrees, fifty 
degrees ; and at the pole it would be ninety degrees, 
comprehending the entire visible heavens, every star 
above the horizon revolving in a circle parallel to it. 

44. Let us now direct our attention to the stars 
towards the south pole, our place of observation 
being the northern hemisphere. In this direction 
the axis of the heavens is depressed below the hori- 
zon, the south pole of the heavens being as far below 
the horizon as the north pole is above it. The centres 
of the circles of daily motion described by the stars 
being in this region below the horizon, the arcs they 
pass through above the horizon are less than semi- 
circumferences, 1 growing smaller and smaller the far- 
ther to the south a star is situated. Their periods of 
visibility will decrease in like manner, until we arrive 
at a point in the southern heavens where a star just 
glimmers for a moment upon the horizon and then 
sets again. 

45. Circle of Perpetual OccuLTATioisr. The stars 
that are situated at a less distance from the south 
pole of the heavens than the pole is depressed below 

1. A semi-circumference is half a circumference. 

State what is respecting the extent of the arc described by stars south of the 
equator, and of the extent of their times of visibility. 

What, js meant by the term circle of pp.rpetual occupation ? How does the circle 



ASTRONOMY. 35 



the horizon, will never in their daily revolution come 
into sight. A circle around the depressed pole, having 
a radius equal to the distance of this pole*below 
the horizon, is called the circle of perpetual occulta- 
Hon, because the stars within it never rise to our 
view. 

46. Like the circle of perpetual apparition, that of 
occultation varies with the variation of latitude, and 
at the same place the magnitude is the same, since 
one pole is elevated the exact amount that the other 
is depressed. Thus, in north latitude ten degrees, the 
south pole of the heavens is ten degrees below the 
horizon, and the radius of the circle of perpetual 
occultation is also ten degrees. In north latitude 
fifty degrees, it is fifty degrees, and at north latitude 
ninety degrees, that is, at the north pole of the earth, 
it comprises the entire half of the heavens below the 
horizon. 

47. We have thus far described the changing as- 
pect of the heavens, by supposing a traveller to pro- 
ceed from the equator towards the north : were he to 
take the opposite direction and move towards the 
south, the phenomena we have described would be 
exactly the same, only reversed in position. Thus, 
the plane of the horizon would dip towards the south, 
the north pole of the heavens would be • depressed, 
the southern elevated, and the stars would be longer 
above the horizon south of the equator than north of 
it. To an observer at the south pole of the earth, 

of perpetual occultation compare in extent with that of perpetual apparition? 
What would be their extent to an observer at either pole of the earth ? State 
what is said respecting the phenomena of tbe heavens when the observer advances 
south of the equator. 



36 



ASTRONOMY. 



the south pole of the heavens would be in the zenith, 
and the circles of daily motion would be parallel to 
the horizon. The circle of perpetual apparition would 
be around the south pole of the heavens, and that of 
occultation about the north, and so on 

48. These remarks may be still farther impressed 
upon the mind by studying the annexed figure, where 




VARYING ASPECT OF THE HEAVENS, ARISING FROM CHANGES IN LATITUDE. 

the outer starred circle represents a section of the 
concave sphere of the heavens, C the earth ; SP and 



Explain the figure. 



A S T R N M Y. 37 



NP its north and south poles ; the line SPNP its axis 
of rotation ; and EQ its equatorial diameter. S ] P X 
and N^P 1 are the north and south poles of the heav- 
ens, and the imaginary line, S^NT 1 , the axis of the 
heavens, about which the stars apparently revolve. 
E^ 1 is the diameter of the celestial equator. 1 1, 1 ; 
2, 2 ; 3, 3, &c, are the diameters of other circles, in 
the circumferences of which the stars appear daily to 
revolve. 

49. Aspect of the Heavexs at the Equator. If a 
spectator is at the equator, at E, his sensible horizon 
coincides with his rational, S^NT 1 , at the vast dis- 
tance of the fixed stars ; and the poles of the heavens 
are, consequently, upon his sensible horizon. Thus 
situated, we see that the circles of daily motion are 
perpendicular to his horizon, and each of the stars 
that are seen at all, apparently describes a semi-cir- 
cumference above and a semi-circumference below 
the horizon, being for twelve hours visible and for 
twelve hours invisible. 

50. At North Latitude Forty Degrees. If the 
observer moves to O, north latitude forty degrees, 
LM becomes his rational horizon The north pole of 
the heavens is elevated, and the south depressed forty 
degrees. The radius of the circle of perpetual appa- 
rition is MR, whose angular breadth is also equal to 
forty degrees ; and LV, having the same extent, is the 
radius of the circle of perpetual occupation. The 
circles of daily motion are here oblique to the hori- 
zon, LM, and the stars north of the equator are con- 
sequently above the horizon a proportionally longer 

1. See Art. 57. 



38 ASTRONOMY. 



time than twelve hours, as they are nearer the circle 
of perpetual apparition. South of the equator they 
are above the horizon for a proportionally shorter 
space than twelve hours, the nearer they approach 
the circle of perpetual occultation. These facts are 
evident when we compare the parts of the lines, 1, 1 ; 
2, 2 ; 4, 4 ;■ and 5, 5, which are above the horizon, LM, 
with the parts that are below, viz., 55, 55 ; 2d, d2, &c. 

51. At the North Pole. At the north pole, NP, 
the horizon takes the direction of the line E^ 1 , the 
north pole of the heavens, N^P 1 , is in the zenith, and 
all the stars in the hemisphere, EHf^PiQ 1 ^ revolve in 
circles parallel to the horizon. E^ 1 is at once the 
radius of the circle of perpetual apparition and occul- 
tation, since all the stars above the horizon never set, 
and those below it never rise above it. If the observer 
moves toward the south pole of the earth, it is clearly 
seen that these appearances are exactly reversed. 

52. Latitude of any Place equal to the Eleva- 
tion of the Pole of the Heavens. From what has 
been just stated, it is evident that the latitude of any 
place is equal to the altitude of the pole of the heav- 
ens above the horizon. For we have seen that at the 
equator, where the latitude is nothing, the elevation 
of the pole is nothing ; at latitude forty degrees the 
elevation of the pole is forty degrees, and at the poles 
of the earth, or latitude ninety degrees, the pole of 
the heavens is ninety degrees from the horizon, and is 
in the zenith. And the same is true for every lati- 
tude, either north or south of the equator. 

To what is the latitude of any place equal ? 



ASTRONOMY. 39 



CHAPTEE III. 

ON THE MODE OF DETERMINING THE PLACE OF A 
HEAVENLY BODY. 

53. The first object of the geographer in describing 
the earth with its kingdoms, cities, mountains, oceans, 
seas, islands, &c, is to determine their exact position 
on the surface of the globe. This he obtains in the 
case of a city, for instance, by finding first, how many 
degrees, minutes and seconds, it is situated east or 
west from a great circle, called a meridian, 1 passing 
through the poles of the earth and some assumed 
point on its surface, as a celebrated observatory ; and 
secondly, its distance in degrees, minutes and seconds, 
north or south of the great circle called the equator, 
passing through the centre of the earth at right an- 
gles to its axis of rotation. Thus, for instance, the 
position of New York City Hall is fixed by finding 
first, that it is situated seventy-four degrees, and three 
seconds (74° 00' 03") west of the meridian passing 
through Greenwich Observatory. This is its longi- 
tude. Next, that it is distant north of the equator 
forty degrees, forty-two minutes, and forty-three se- 
conds (40° 42' 43"). This is its latitude. These two 
measurements are sufficient to mark with precision its 
situation upon the globe ; for no other spot on its sur- 
face can have this latitude and longitude. 

I. See Art. 55 for the meaning of the term meridian. 

What is the subject of Chapter III. ? What is the first object of the geographer ? 
In what manner does he determine the position of a city ? Give an instance. 



40 ASTRONOMY. 



54. In a similar way the astronomer determines the 
position of stars in the concave sphere of the heav- 
ens, by measuring their angular distances from the 
planes of two great circles, at right angles to each 
other. And for this purpose he supposes the globe 
and sky to be intersected by certain lines and circles 
which we will now describe. 

55. Celestial Sphere, Poles, Axes, and Meri- 
dians. The celestial sphere is the concave sphere of 
the heavens, in which the stars appear to be set. The 
poles of the earth are the extremities of that imagi- 
nary line upon which it revolves : the latter is called 
the axis. If any plane passes through the poles and 
the axis in any direction, its intersection with the 
surface of the earth is a circle, and is called a terres- 
trial meridian. Thus, in Fig. 16, which represents 
the earth and the celestial sphere, the line JSTS is the 
axis of the earth ; N the north pole ; S the south 
pole ; and NES, N1S, N2S, N3S, are terrestrial me- 
ridians. 

The axis of the earth, extended in imagination each 
way until it meets the starry sky, becomes the axis 
of the heavens, or celestial sphere, around which all 
the stars appear to revolve. The extremities of this 
axis are the poles of the heavens. Thus, in the figure, 
where the outer starred circle represents a section of 
the celestial sphere, the line lOS 1 is the axis of the ce- 
lestial sphere, and N 1 and S 1 its north and south poles. 



How does the astronomer determine the position of a star ? What is meant by 
the celestial sphere? The poles of the earth? Its axis ? Terrestrial meridians ? 
Explain from figure. What is meant by tl^fi axis of the celestial sphere ? The 
poles of the heaven?? 



41 



A S T R N O M Y 



FIG. 16. 




THE EARTH AND THE CELESTIAL SPHERE. 

56. If any plane passes through the poles and axis 
of the heavens in any direction, its intersection with 
the imaginary surface of the celestial sphere is a celes- 
tial meridian. Thus, N'E'S', W1S\, MS*, and ISPBS 1 , 
are celestial meridians. 



What is meant by celestial meridians ] Explain from figure. 



42 ASTRONOMY. 



57. Equators. If we suppose a plane passing through 
the centre of the earth, perpendicular to the axis of 
rotation, its intersection with the surface of the earth 
forms a circle called the equator, or terrestrial equator * 
and if this plane is extended in imagination to the 
fixed stars, its intersection with the celestial sphere is 
also a circle, called the celestial equator, or equinoctial. 
Thus, in Fig. 16, EQ is the equator, and E^ 1 the ce- ( 
lestial equator. They appear as straight lines in the 
figure, because we see them in the direction of their 
planes. 

58. Vertical Circles. Vertical circles are those 
which are imagined to be formed by planes passing 
through the zenith, perpendicular to the horizon, and 
intersecting the celestial sphere. The vertical circle 
passing through the east and west points of the hori- 
zon is termed the prime vertical, while that which in- 
tersects the north and south points becomes a meri- 
dian. Thus, in Fig. 17, where A represents the earth, 
SZWMN the celestial sphere, Z the zenith, and the 
plane SWNE the horizon — PZHM is a vertical 
circle, WZEM the prime vertical, and SZNM a meri- 
dian. 

59. The Position of a Star— how determined. The 
place of a star in the sky may be determined in three 
ways. First, by referring it to the planes of a celes- 
tial meridian and of the horizon. Secondly, by no- 
ting its distance from the planes of a given meridian 



What is the terrestrial equator? What the celestial or equinoctial ? What are 
vertical circles ? What the prime vertical 1 Is a meridian a vertical circle ? Ex- 
plain from figure. In how many ways is the position of a star fixed ? Describe 
them 



ASTRONOMY 



43 



FIG. 17. 




AZIMUTH AND ALTITUDE OF A STAR. 



and the celestial equator. Thirdly, by referring it to 
the planes of a given meridian and the ecliptic. 1 

60. Azimuth akd Altitude of a Star. The azimuth 
of a star is its angular distance from a meridian mea- 
sured on the horizon, and its altitude is its distance 
from the horizon measured on a vertical circle pass- 
ing through the star. Thus, in Fig. 17, A represents 
a place on the earth ; Z the zenith of the place where 
an observer is stationed ; NEHRW the circle of the 
horizon ; SZNM the meridian circle ; B a star, and 
ZBHMP a vertical circle passing through the zenith 
and the star B : all these circles being circles of the 

1. For the meaning of the term ecliptic, see Art. 63. 



"What is meant bv the azimuth and altitude of a star ? 



44 



ASTRONOMY 



celestial sphere. Then the angle NAH is the azi- 
muth, and BAH the altitude. If the star is in the 
south, as at P, its azimuth would be reckoned from 
S, and would be SAR : ZAB is the zenith distance of 
the star. 

61. Declination and Eight Ascension. The angu- 
lar distance of a star, measured from the celestial 
equator, on a meridian passing through the star, is 
called its declination. Right ascension is the distance 
of a star measured on the celestial equator in an 
easterly direction from the meridian passing through 
the vernal equinox, 1 the place where the sun meets 
the celestial equator in the spring, and is so called be- 
cause when the sun appears at this point in the heav- 
ens, the nights, and consequently the days, are equal 
in length in every part of the world. 

62. The subject is illustrated by Fig. 18, where P 

FIG. 18. 




DECLINATION, RIGHT ASCENSION, LATITUDE, LONGITUDE. 



1. Ver, the Latin word for spring ; equinox, a word formed from two 
Latin words, (zquus, equal, and nox, night. 

Show from, the figure how these measurements of a star are taken. What is 
meant by zenith distance ? What is declination and right ascension ? 



ASTRONOMY. 45 



represents the north pole of the heavens, P T 1 a celestial 
meridian passing through the vernal equinox ; QAQ 
the celestial equator ; S the place of a star ; PSA 
a part of a celestial meridian passing through the star ; 
and C the centre of the celestial sphere ; or, what is 
the same in effect, the place of the spectator. 

Now the declination of the star is the arc SA, since 
this arc measures the angular distance of the star from 
the equator QAQ 1 , and if AS contains forty degrees, 
the declination of the star is forty degrees. The right 
ascension of the star is TA, and if this arc contains 
fifteen degrees, the star at S has fifteen degrees of right 
ascension. 

63. Ecliptic. The imaginary line that the earth de- 
scribes in her annual progress around the sun is 
termed her orbit, and its plane passes through the 
centre of the earth and sun, having an inclination to 
that of the celestial equator of about 23° 27'. Its. 
intersection with the celestial sphere is called the 
ecliptic? and constitutes what may be regarded as a 
great circle of the heavens. 

The angular distance of a star from the ecliptic, 
measured on a great circle passing through the poles 
of the ecliptic, is called its latitude, and its angular 

1. The character °P is called Aries, and is that point in the celestial 
equator which is termed the vernal equinox. P HP is read thus, P, 
Aries. 

2. So called because eclipses happen when the sun, earth and moon 
are in its plane. 



Show from figure what is the declination and right ascension of the star at S. 
What is meant by the earth's orbit ? What is the inclination of its plane to that 
of the celestial equator 1 What is the ecliptic 1 What is the latitude of a star ? 



46 ASTRONOMY. 



distance, measured on the ecliptic, eastward from 
Aries, is termed its longitude. Thus, in Fig. 18, where 
TLE represents the ecliptic ; P 1 its north pole ; P 1c p 
a great circle passing through Aries ; and P^L, a 
great circle passing through the star at S X SL, is the 
latitude of the star, and tL the longitude of the star, 
being the angular distance from Aries measured on 
the ecliptic to the great circle P X SL passing through 
the star. 

64. The Signs. The ecliptic is divided in twelve 
equal parts, called signs, each sign occupying in the 
heavens an extent of thirty degrees. Within these 
divisions are situated certain conspicuous clusters of 
stars, termed constellations, which, in the infancy of 
Astronomy, received particular names, and these 
names were also given to the signs. The following 
are the names and characters of the signs, north of 
the celestial equator, beginning at the vernal equinox : 

ARIES, The Ram, T CANCER, The Crab, S 

TAURUS, The Bull, » LEO, The Lion, SI 

GEMINI, The Twins, n VIRGO, The Virgin, flJJ 

The next six, the names and characters of those 
south of the celestial equator : 

LIBRA, The Scales, =^ CAPRICORNUS, The Goat,. . ..V5 

SCORPIO, The Scorpion, TTI AQUARIUS, The Water-Bearer,^? 

SAGITTARIUS, The Archer, t PISCES, The Fish, X 

65. Zodiac. The zodiac is a belt of the celestial 
sphere, extending eight degrees on each side of the 
ecliptic. It is so called from the Greek word zodia, 

What its longitude ? Explain from figure. How is the ecliptic divided ? What 
is the extent of each sign ? What are situated within these divisions ? Give the 
names of the signs. Which are north, and which south, of the celestial equator? 
W h at is the zodiac, and why is it so called ? 



ASTRONOMY. 47 

meaning figures of animals, because the signs of the 
ecliptic are formed principally of the figures of ani- 
mals. 



CHAPTEE IV. 

of the measurement of time. 

66. Of the Time occupied by the Earth in perform- 
ing one Rotation. — How determined. This is ascer- 
tained by means of an accurate clock, and a transit 
instrument (Fig. 19), which is a telescope placed in 

FIG, 19. 



TRANSIT INSTRUMENT. 



Of what does Chapter IV. treat ? How can we ascertain the time occupied by 
the earth in performing one rotation ? Describe the transit instrument. 



48 ASTRONOMY. 



the plane of the meridian; so that when a person 
looks through it he looks either in a north or south 
direction. 

Within this instrument is placed a system of wires 
like those shown at ac (Fig. 20), one horizontal and 
live vertical : the latter being parallel to each other, 
and separated by equal intervals. 

FIG. 20. 




WIRES. 



67. Let us now observe the way in which the as- 
tronomer ascertains the time of the rotation of the 
earth on its axis. Seated in his observatory, with his 
telescope and clock properly adjusted, he selects for 
his sky-mark some bright fixed star near the meri- 
dian. He watches it closely, and soon the earth, as 
it rotates towards the east, brings the telescope up to 
the star. At the moment the latter is upon the me- 
ridian, the middle vertical wire of the instrument 
cuts the star exactly in two, and the astronomer notes 
the time by his astronomical clock : we will suppose 
it to be eight. During the rest of the night and the 
succeeding day, the astronomer, with his observatory 
and instruments rotating with the earth, passes star 



ASTRONOMY. v 49 



after star in succession, and as eight o'clock approaches, 
the observed star of the preceding evening is seen 
again near the meridian. 

68. At eight o'clock precisely the central vertical 
wire again cuts the star exactly in two, showing that 
the earth has completed one rotation. Twenty-four 
hours have elapsed since the first observation : this, 
then, is the period of time occupied by the earth in 
performing one entire rotation. Such observations 
have been made repeatedly, both upon the same star 
and upon different stars, and at stations widely sepa- 
rated, and the result has been found to be invariably 
the same. Centuries may intervene between two 
series of observations^ and yet the results are identi- 
cal : we thus arrive at the conclusion that the interval 
of time elapsing between two successive transits^ of a 
fixed star, and which measures one entire revolution 
of the earth, is unchangeably the same. 

69. Standard Unit of Time. The period of the 
earth's rotation on its axis is the universally acknowl- 
edged unit of time, since it is the only natural marked 
division of time which continues unaltered from age 
to age. The different periods of time in common use 
all date from this. Weeks, months, and years, are 

1. Transit. The transit of a star is the moment of its passage across 
the meridian when it is cut exactly through the centre by the central 
vertical wire of the transit instrument. Transit, from the Latin word 
transitus, a passage. 



Describe in full how the time occupied by the earth in performing one rotation 
is determined. Is this period changeable ? 

What is the standard of time? Why is this division of time adopted as a stand- 
ard ? What is said of weeks, months, and years ? 

3 



50 ASTRONOMY 



reckoned by days and fractions of a day, while hours, 
minutes, and seconds, are divisions and sub-divisions 
of the day. 

70. Of the Sidereal and Solar Day. The sidereal 1 
day is the length of time that elapses between two 
successive transits of the same fixed star across the 
meridian — in other words, the period of the earth's 
rotation. The solar 2 day is the time that elapses at 
any place between noon of one day and noon of the 
next. The solar day is about four minutes longer 
than the sidereal ; and the cause of this difference 
we will now proceed to explain. 

71. The subject is illustrated by the following dia- 
gram (Fig. 21), where S represents the sun, and E, E 
the earth in two positions of its orbit ; the dark semi- 
circles are sections of the hemispheres unenlightened 
by the sun, and the light semicircles sections of the 
enlightened hemispheres. In position 1, it is noon at 
N, because there are equal portions of the illumined 
hemisphere on the east and west side of it. But on the 
next day, when the earth has made one complete ro- 
tation, and has in the meanwhile also moved along 
in its orbit, CD, to position 2, it will not then be noon 
at N, for the meridian plane now passes through N 1 : 
the earth will have to revolve on its axis until N has 
arrived in the position, N 1 , before it will be noon at 
N, and the time occupied in describing the arc NN 1 , 
will be the excess of the solar above the sidereal day. 

1. Sidereal, from sidera, the Latin word for stars. 

2. Solar, from sol, the Latin word for the sun. 

What is said of hours, minutes, and seconds? What is meant by the term side- 
real day? What by solar day? How much longer is it than the sidereal day * 
Explain by Fig. 21. 



ASTRONOM Y 



51 



FIG. 21. 




SOLAR AND SIDEREAL DAY. 



72. The difference in the length of the solar and 
sidereal day may be explained by the motions of the 
hands of a watch. Calling the time made by one 
revolution of the minute hand a sidereal day, a solar 
day may be compared to the extent of time that 
elapses from the instant the hour and minute hands 
are together, to the next time they are again in that 

Explain the difference between solar and sidereal time. Illustrate by the mo- 
tions of the hands of a watch. 



52 ASTRONOMY. 



position — a period manifestly longer than the first, 
since the minute hand has not only to make one revo- 
lution, but must also catch up with the hour hand, 
which has all the while been advancing. 

The difference between the solar and sidereal day 
is not always the same ; the solar days are, therefore, 
of unequal length. 

73. Apparent Time. Apparent time is the time 
computed from noon to noon ; that is, in solar days. 

74. Mean Solar Time. Mean solar time 1 is an arbi- 
trary division of time, in which all the solar days are 
supposed to be of the same length, this length being 
found by dividing the whole amount of time in a solar 
year by the number of solar days in that period. Days 
of changing length would furnish an inconvenient 
method of reckoning for mankind ; mean solar time is 
therefore employed in the common affairs of life, and 
constitutes civil time. 2 

The civil day commences at twelve o'clock at night, 
and is divided into two periods, of twelve hours each, 
reckoning from one to twelve from midnight to noon, 
and again from one to twelve from noon to midnight. 

75. Astronomical Time. Astronomical time is ap- 
parent time, and is employed for scientific purposes. 
The astronomical day commences at noon, and ends at 

1. Mean solar time. The word mean here signifies average. 

2. Civil time, the legal time, or that appointed by a government to 
be used in their dominions. 

Are the solar days of equal length ? What is apparent time? What is mean 
solar time ? What is civil time ? Why is mean solar time adopted as civil time ? 
When does the civil day begin, and how is it reckoned? What is astronomical 
time ? When does the astronomical day begin ? Of how many hours does it con- 
sist, and how is it reckoned ? 



ASTRONOMY. 53 



noon on the next day. It consists of twenty-four 
hours, the hours being counted from one to twenty- 
four. 

76. The Tropical Tear. The length of time employed 
by the earth in performing an entire circuit from any 
point in the ecliptic ', as the summer solstice, 1 to the 
same point again, constitutes a tropical 2 year, which 
contains three hundred and sixty-five days, five hours, 
forty-eight minutes, and forty-seven eight-tenths se- 
conds (365d. 5h. 48m. 47.8sec.) The fractions of a 
day belonging to a year of this length would be 
manifestly inconvenient for the purposes of society, 
and for this reason the civil year is made to consist 
of three hundred and sixty-five entire days. 

All nations who have made any progress in the art 
of computing time, have regarded the civil year as 
consisting of an even number of days. They have 
made, for the most part, however, at stated intervals, 
such corrections, that the real position of the earth in 

1. The solstices are the two points in the ecliptic farthest from the 
equator. The sun is at the summer solstice about the 22d of June, and 
at the winter solstice about the 22d of December. These points are so 
called from two Latin words, sol, the sun, and stare, to stand ; because 
the sun appears to stand still at these points for a short time, before it 
turns back in its apparent course towards the equator. 

2. Tropical year, so called, from the Greek word trepo, to turn, be- 
cause the sun reverses its apparent course upon arriving at either sol- 
stice. In our summer, after advancing apparently as far north as the 
summer solstice, it then turns back to the south, and in winter, after 
retreating as far south as the winter solstice, it turns back to the north. 

What is a tropical year? What is its length ? What is the length of a civil 
year ? Why is not the tropical year employed as the civil year ? What has been 
the custom of all nations who have possessed a knowledge of the computation of 
time, in regard to the civil year? 



54 ASTRONOMY. 

its orbit shall on the whole correspond with the posi- 
tion indicated by any date in the year. 

77. A moment's reflection will show the necessity 
of such corrections. Four civil years are shorter 
than four tropical years by nearly one day (4 x 5h. 
48m. 47.8"), so that in every four years about one day 
would be lost in the reckoning. For if the reckon- 
ing commenced at the day of the summer solstice, 
on the 22d of June, four years afterwards, the earth 
would not have arrived at the solstice by a day's 
journey, and the solstice would take place on 
the 23d. In four years more it would happen on 
the 24th, and in four more on the 25th, and so on. 
This mode of reckoning, if continued uncorrected, 
would thus in course of time make either solstice, or 
any other position of the earth in its orbit, occur suc- 
cessively on every day of the civil year. We should 
have, therefore, at times, the summer in the winter 
months, and the winter in the summer months. 

78. Egyptians. The ancient Egyptians regarded 
the civil year as consisting of 365 days. They made 
no corrections, and suffered their festivals, though 
recurring at the same date, to run through the entire 
natural year. 

79. Mexicans. The Mexicans regarded the year as 
consisting of three hundred and sixty-live days, but 
made a correction of thirteen days for one period of 
fifty-two years, and twelve for the next, amounting 
to a correction of tioentyfive days for every one hun- 



Supposing the year to consist of three hundred and sixty-five days only, what 
woald happen if no corrections were made ? Of how many days did the civil year 
consist among the ancier.t Egyptians? How was it computed by the Mexicans? 



ASTRONOMY. 55 



dred and four years. The accuracy obtained by this 
method is truly surprising for the excess of the actual 
over the civil year ; viz., five hours, forty-eight min- 
utes, and forty-seven eight-tenths seconds, multiplied 
by one hundred and four, gives as a product twenty- 
Jive days, four hours, thirty four minutes, and fifty- 
one seconds, the error of reckoning in a century being 
only about four and a half hours. 

80. The calendar in use among Christian nations is 
derived from the Romans. The civil year is here 
made to consist of three hundred and sixty-five days, 
the necessary corrections being applied at stated in- 
tervals. The first correction in this calendar was 
made by Julius Caesar, and the rule was adopted of 
adding one day to every fourth year, by giving 
February twenty-nine instead of twenty-eight days. 
This fourth year, consisting of three hundred and sixty- 
six days, is called the Bissextile or leap-year. 

81. But the Julian correction was too great, be- 
cause the year was thereby assumed to be three hun- 
dred and sixty-five days and six hours long, when, in 
fact, it is about eleven minutes shorter — an error which, 
in the course of nine hundred years, would amount to 
very nearly seven days. 

82. In the year 1582, it had amounted to about ten 
days, and a reform was made by Pope Gregory XIII. 
The remedy was obvious, and consisted in omitting 
ten nominal days, calling the day next succeeding the 
4th of October the 15th, instead of the 5th. This 



What is said respecting accuracy of their correction ? How much did the error 
amount to in 1582 ? By whom was a reform made 1 How was the error corrected ? 



56 ASTRONOMY. 



change was made at once in all Catholic countries, 
but was not adopted in England until the year 1752, 
by which time the error had amounted to eleven clays. 
The change of style, as it is termed, was there effected 
by an Act of Parliament, decreeing that the day after 
the 2d of September, old style, should be called the 
14th, which was the first day of the new style ; and 
by the same authority, the year, which before had 
begun on the 25th of March, was made to begin on 
the 1st of January. This latter change was accom- 
plished by making the preceding year (1751) to con- 
sist of nine months only, causing it to end at the 
beginning of the 1st of January, instead of the 25th 
of March. The year 1752 commenced on the 1st of 
January. 



CHAPTEE V. 

OF THE EARTH'S ORBIT, AND THE SEASONS. 



83. The Earth's Orbit. Astronomers have proved 
that the earth revolves about the sun. The path it 
describes is called its orbit. This orbit is not a circle, 
but an ellipse, the sun being situated in one of the foci. 
The average distance of the earth from the sun is 
95,000,000 of miles ; the extent of its orbit, 600,000,- 
000 ; and through this immense space the earth sweeps 

Where was the change at once adopted 1 When introduced into England ? 
What was then the amount of error ? How was the change of style effected, and 
what alterations were made in the calendar ? How was the second change accom- 
plished ? What is, the subject of Chapter V. ? What have astronomers proved ? 
What is the earth's distance from the sun ? What the extent of its orbit ? 



ASTRONOMY. 57 



in the course of a year at the rate of 19 miles per 
second. The earth is kept in its path by the attract- 
ive force of the sun, and its different positions in 
respect to the latter during its revolution, give rise to 
the seasons of the year. 

84. The Seasons. The changes of the seasons de- 
pend upon three causes. First, the fact that the sun 
illumines but one half of the earth at a time ; Second- 
ly, that the axis on which the earth revolves is in- 
clined to the plane of the ecliptic ; Thirdly, that its 
position at any one point in the earth's orbit is inva- 
riably parallel to its position at every other point. 

85. By the aid of Fig. 22, 1 we shall be enabled to 
perceive how the variety of the seasons is produced 
by the causes just mentioned. In this cut, S 1 repre- 
sents the sun, the twelve globes indicate the several 
positions of the earth in its orbit, in the successive 
months of the year with the corresponding signs, and 
the dotted line CS ] C gives the direction of the plane 
of the ecliptic. In the several globes C is the centre 
of the earth, DCL is an equatorial diameter, and 
shows the direction of the plane of the equator ; the 
diameter at right angles to this, viz., NCS, is the 
axis of the earth, and its extremities the north and 
south poles — N representing the north pole. The tw r o 
large arcs of circles on each side of DCL are the 



1. The figure is here drawn as if the plane of the ecliptic was viewed 
obliquely ; the orbit of the earth, therefore, appears more eccentric than 
it actually is. 



Upon how many causes io the changes of the seasons depend ? Name them. 
Explain the figure. 



58 



A S T ft N M t 



FIG. 22. 




ASTRONOMY. 59 



tropics, and the small arcs near the poles the arctic* 
(northern) and antarctic 2 (southern), or polar circles. 
The lines drawn in each globe from C, parallel to 
CS 1 ^ indicate the position of the plane of the ecliptic 
with respect to that of the equator. 

86. Spring. At the vernal equinox (March), w^hen 
the earth is in Libra, 3 the circle of illumination ex- 
tends to the two poles f the sun is in the plane of the 
equator, and is seen from the earth in this plane. As 
the earth rotates on its axis, every point upon its sur- 
face is then half the time of one rotation in darkness, 
and the other half in light In this position of the 
earth, the days and nights are therefore equal all 
over the globe. 

87. Summer. "When the earth is in Capricorn at 
the northern summer solstice (June), the axis being 
unchanged in direction, the north pole is presented 
towards the sun, and the circle of illumination extends 

1. Arctic (northern), from the Greek word arktos, meaning bear, 
because the north pole of the heavens is in the constellation called the 
bear. 

2. Antarctic, from the Greek and, opposite, and arktos, bear : i. e., 
south. 

3. At the time of the vernal equinox the earth is in Libra, but the 
sun, as viewed from the earth, appears on the opposite side of the heavens, 
in the sign Aries. 

4. In the figure, at the vernal equinox, the dark hemisphere of the 
earth is presented to our view, the illuminated hemisphere being toward 
the sun, as shown in the globe at Aries. The circumference of the 
circle of illumination, both at Libra and Aries, is DNLS 

At the time of the vernal equinox, what is the position of the circle of illumi- 
nation in respect to the poles? In what plane is the sun then situated, and in 
what plane seen ? What is said in regard to the length of the days and nights 
at this time? What is the position of the circle of illumination at the northern 
summer solstice ? 



60 ASTRONOMY. 



beyond the pole, 1ST, to the arctic (northern) circle, while 
in the southern hemisphere it falls short of the south 
pole, S, reaching only to the antarctic (southern) circle. 
The snn is now seen from the earth in the direction 
CS l , having apparently moved toward the north the 
extent of the angle DCS 1 . This angle DCS 1 measures 
the inclination of the plane of the ecliptic to that of 
the equator, which is termed its obliquity, and is equal 
to about twenty-three and one half degrees (more 
nearly 23° 27' 38"). 

88. The exact distance that the circle of illumina- 
tion now overlaps the northern and falls short of the 
south pole, is equal to the obliquity of the ecliptic/ 
for since the time of the vernal equinox, the sun in 
his apparent motion has departed from the plane of 
the equator at the same rate that the plane of the 
circle of illumination has departed from the poles. 
The parallels of latitude, therefore, to which the circle 
of illumination extends at the summer solstice, and 
which are termed the arctic and antarctic circles, are 
each about twenty-three and a half degrees from their 
respective poles. The regions inclosed within these 
circles are called the frigid zones. 

89. At the time of the northern summer solstice, 
continual day reigns at all those places that are 
situated within the arctic circle, inasmuch as the 
daily rotation of the globe does not carry them with- 
out the circle of illumination / while over the regions 
that lie within the antarctic circle, an unbroken night 

What is the obliquity of the ecliptic ? Its extent 1 What is the extent of the 
arctic and antarctic circles, and why ? What are the frigid zones ? Where does 
continual day prevail at the time of the northern summer solstice, and why ? 
Where unbroken night, and why ? 



ASTRONOMY. 61 



prevails, because the earth in its rotation does not at 
this time bring them within the circle of illumination. 
It is evident, from an inspection of the figure, that in 
the northern hemisphere, since half the axis, CN", falls 
within the plane of the circle of illumination, that 
the days will increase in length, and the nights de- 
crease from the equator to the arctic circle, where 
there exists a continual day. In the southern hemi- 
sphere, since half the axis, CS, falls without the plane 
of the circle of illumination, the days will decrease 
and the nights increase in length from the equator to 
the antarctic circle, where an uninterrupted night pre- 
vails. 

90. At the vernal equinox, the days and nights, as 
we have seen, are equal in length. A difference in 
this respect commences as soon as the earth departs 
from this point, which gradually increases up to the 
time of the summer solstice, when the difference in 
the lengths of the days and nights is greatest. 

91. At the summer solstice, the sun's rays fall per- 
pendicularly upon the surface of the earth in the 
direction 8*0, at a point about twenty-three and a 
half degrees (23° 27' 38") north of the equator; 
the parallel of latitude passing through this point is 
termed the northern tropic, or tropic 1 of Cancer, be- 

1. Tronic, derived from the Greek word trepo, to turn about, because 
when the sun, in its apparent advance to the north, has arrived at a point 
about twenty-three and one half degrees from the equator, it then turns 
about and moves toward the south. 

What is said respecting the lengths of the days and nights in the northern 
hemisphere ? In the southern ? When do these differences in length begin ? 
When greatest ? How is the position of the northern tropic determined ? 



62 ASTRONOMY 



cause the sun as now seen from the earth appears in 
the sign Cancer. 

92. Autumn. As the earth departs from the north- 
ern summer solstice, and by degrees comes round to 
the autumnal equinox (September), the circle of illu- 
mination gradually approaches the poles, shortening 
the days, and lengthening the nights in the northern 
hemisphere, and producing the contrary effects in the 
southern. When the earth has arrived at the autum- 
nal equinox in the sign Aries, the circle of illumina- 
tion again passes through both poles, and the days 
and nights are once more equal in length. 

93. Winter. The earth, moving onward in its 
course toward the northern winter solstice, the circle 
of illumination also changes its position, falling short 
of the north pole more and more, and gradually ex- 
tending beyond the south pole—increasing the dura- 
tion of the nights in the northern hemisphere, and 
diminishing that of the days ; while in the southern 
hemisphere the opposite effects are produced. At 
the winter solstice, 'when the earth is in the sign 
Cancer (December), this change has reached its full 
extent ; the circle of illumination then reaches be- 
yond the south pole to the antarctic circle, and the 
regions within this circle now enjoy a continual day. 
But in the northern hemisphere the circle of illumina- 
tion extends only to the arctic circle, and the space with- 
in the latter is now overshadowed by a constant night. 

What is it called? What changes take place as the earth moves toward the 
autumnal equinox ? What is said of the circle of illumination, and of the days and 
nights at the equinox? Describe the changes that occur as the earth moves toward 
the northern winter solstice. At the northern winter solstice, what is said in refer- 
ence to the circle of illumination, and the lengths of the days and nights ? Where 
•jeos there now reign an unbroken dav ? Where an unbroken ni<?ht ? 



ASTRONOMY. 63 



94. As the earth withdraws from the northern 
winter solstice, and again returns to the vernal 
equinox, the circle of illumination by degrees again 
approaches the poles, and the differences between the 
lengths of the days and nights grow less and less 
until they cease to exist, when the vernal equinox is 
attained. 

95. A glance at the figure shows us that the sun at 
the northern winter solstice is seen south of the equa- 
tor in the direction CS 1 . And it is seen at this point 
as far south of the equator as it was north at the time 
of the northern summer solstice, viz., 23° 27' 38". 
The circle of illumination, therefore, at the two sol- 
stices, in turn, overlaps and falls short of the same pole 
the same extent of space. 

96. The place where the line S*C falls upon the 
surface of the earth south of the equator, is the place 
of that parallel of latitude, which is termed the 
southern tropic, and which is about twenty-three and 
a half degrees (23° 27' 38") south of the equator. 
It is called the tropic of Capricorn. 

97. That portion of the surface of the earth in- 
cluded between the northern and southern tropics 
is called the torrid zone, and those parts that 
lie between the two tropics and the arctic and 
antarctic circles, the north and south temperate 

ZONES. 



What changes take place as the earth returns to the vernal equinox? How far 
south of the equator is the sun seen at the northern winter solstice ? How much 
does the circle of illumination at the two solstices overlap and fall short of ihe 
snme pole? How is the position of the southern tropic determined? What is its 
extent ? What is it ca.lled ? What is meant by the torrid zone ? What by the 
temperate zones ? 



64 ASTRONOMY. 



98. We must bear in mind, in this explanation, 

that when it is winter in the northern hemisphere 

it is summer in the southern, and when it is winter 

in the southern hemisphere it is summer in the 

northern. 

•» » 

99. Polab Winters. From what has just been 
stated, it appears that within the polar circle there 
are long intervals of day and night ; while at the 
poles themselves there is but one day and one night, 
each of six months duration. But several causes 
exist which tend to shorten the dreary winter of the 
frigid zones. The principal of these are refraction 
and twilight. 

"When the sun is a little below the horizon, the rays 
that proceed from it are so bent down (refracted) to- 
wards the earth in coming through the air, that the 
orb is actually seen above the horizon. The refrac- 
tion in the polar regions is very great. 

An extraordinary instance of refraction is said to 
have occurred in the year 1597, at Nova Zembla, in 
north latitude 75^°, the sun appearing above the hori- 
zon, when it was really seven times the length of its 
apparent diameter below it. The effect, therefore, of 
refraction upon the sun is to increase the length of 
the day. The combined effect of refraction and twi- 
light in shortening the polar night is so great that at 
the very poles, its duration is only seventy days in- 
stead of six months / and even the obscurity that then 

What must we bear in mind ? What is evident from the facts that have jnst 
been stated 1 What is the effect of refraction upon the sun ? What extraordinary 
instance of refraction occurred at Nova Zembla in 1597 ? What is said of the com- 
bined influences of refraction and twilight in shortening the polar night ? What 
other mitigating influences exist "? 



ASTRONOMY. £5 



prevails is relieved by the constant presence of the 
moon, when it passes north of the equator ; and like- 
wise by the frequent and fitful splendors of the north- 
em lights. 



PART SECOND, 

SOLAR SYSTEM. 

CHAPTER I. 

THE SUM". 

100. We now proceed to describe the sun, a vast 
luminous and material globe, around which a train of 
planets and comets revolve, constituting with the sun 

the SOLAR SYSTEM. 

101. 'When the sun is observed through colored 
glasses, which intercept a portion of its heat, and 
lessen its dazzling brilliancy, it presents the appear- 
ance of a perfect circle. We are not, however, to 
suppose that it is flat and round like a plate. While 
we revolve on the earth about the sun, the latter at 
the same time rotates on its axis, and yet always ap- 
pears round — a fact which proves it to be a globe like 
our earth, for it is only a spherical body that will appear 
of a circidar form when viewed from any direction. 

102. Diameter. The sun's diameter is 887,036 
miles — nearly one hundred and eleven times greater 
than that of the earth. In Fig. 23, the two circles S 
and E represent the relative magnitudes of the sun 
and earth, the diameter of the larger circle being 111 
times greater than that of the smaller. 

What is the subject of Part SecOxVD ? What of Chapter I. ? What is said of 
the sun ? What form does it present when viewed through colored glasses ? Is it 
flat and round like a plate ? What proof have we that it is a globe? What is the 
length of the sun's diameter? How much larger than that of the earth ? 



ASTRONOMY 




RELATIVE MAGNITUDES OF THE SUN AND EARTH. 



103. Size or Bulk. If we had two cubical boxes, 
A and B, and the length, breadth, and height of A 
were severally 2 feet, while the length, breadth, and 
height of B were each 3 feet, the size of A would be 
found by multiplying 2 into itself twice / thus : 2 X 2 
x 2, the product of which is 8. The size of B would 
be obtained by multiplying 3 in the same manner ; 
thus : 3 x 3 x 3, the product of which is 27. The 

Explain how the si%e of the sun is ascertained. 



68 ASTRONOMY. 



numbers 8 and 27 are respectively the cubes of 2 and 
3, and the size of the boxes A and B have therefore 
the same relations to each other as the cubes of their 
respective heights, lengths, or breadths. 

104. Now mathematicians have proved that the 
sizes of spheres are to each -other as the cubes of their 
diameters. Calling then the diameter of the earth 1, 
and its size 1, and the diameter of the sun 111, the 
following proportion will give us the size of the sun 
compared with that of the earth: 

Cube of the Cube of the Size of Size of 

Earth's diameter. Sun's diameter, the Earth. the Sun. 

1x1x1=1 : lllxlllxlll :: 1 :- 1,367,631; 
the last term being obtained by the common rule of 
three. The sun is thus found to be about one million 
four hundred thousand times (1,400,000) larger than 
the earth. 

105. Quantity of Matter est the Sun. Astrono- 
mers have ascertained from reliable calculations that 
the sun is formed of much lighter materials than the 
earth; so much so, that if four cubic feet of the sun's 
matter at its average density could be transported to 
the surface of our globe, it would weigh but a trifle 
more than one cubic foot of the earffis matter taken 
at its average density. The quantity of matter in 
the sun is, therefore, about 350,000 times (\- of 
1,400,000) greater than the quantity of matter in the 
earth. 

106. Weight of Bodies at the Surface of the Sun. 
A body which weighs 100 pounds on the surface of 

How much larger is it than the earth ? Is the matter of the sun lighter or heavier 
than that of the earth ? How much lighter ? How much more matter is there in 
the sun than in the earth? 



ASTRONOMY. 69 



the earth, would, if transported to the stm, weigh 
nearly 2800 pounds. The weight of a body on our 
globe, or on any other, is a measure of the force with 
which it is drawn toward the centre of the globe y 1 and 
when the globes vary in size, the magnitude of this 
force is dependent upon two circumstances. First, 
the relative quantities of matter in the two bodies ; 
secondly, the comparative distances of the surfaces of 
the globes from their respective centres. 

107. If there were two globes, M and N, and U 
contained ninety times as much matter as M, it would, 
in virtue of this greater amount of matter, draw any 
body placed upon its surface down toward its centre, 
with ninety times more power than if the same body 
was placed on the surface of M. But if the distance 
from N's centre to its surface was three times greater 
than the distance of M's centre from its surface, the 
body placed on N's surface would in virtue of this 
circumstance be drawn toward the centre with nine 
(3 x 3, the square of 3) times less power than when 
placed upon M's surface. By being removed from 
M to N, the weight of the body would therefore be 
increased 90 times, and diminished 9 times, 2 which is 
the same as saying that the weight of the body would 
be increased 10 times. 

1. This force is called the force of gravity. 

2. This rule is technically expressed by saying that the force of gravity 
varies directly as the quantity of matter in the attracting body, and in- 
versely as the square of the distance from its centre. 

If a mass of matter weighed 100 pounds on. the surface of the earth, what would 
be its weight on the surface of the sun ? What is the weight of a body the mea- 
sure of? Upon what two circumstances does this force depend? Give the 
explanation. What is the law respecting this force, in relation to the quantity of 
matter ? What in relation to the distance from the centre to the surface of the 
attracting globe ? 



70 ASTRONOMY. 



108. Now to apply this rule to the sun : If a mass 
of matter which weighs a pound at the surface of the 
earth were to be transported to the surface of the sun, 
its weight would be increased 350,000 times, in conse- 
quence of the greater amount of matter in the sun, 
and diminished 12,321 times (111 x 111), because it 
would be removed 111 times farther from the centre 
of the body on which it then rested than when at the 
earth. Multiplying, therefore, 1 by 350,000, and di- 
viding this product by 12,321, the quotient is 28.4, 
which is the weight in pounds of the given mass at 
the surfs surface. A body, therefore, which weighs 
one hundred pounds at the surface of the earth, would 
weigh about twenty-eight hundred pounds at the sur- 
face of the sun. A person weighing at the earth 150 
pounds, would weigh at the sun nearly two tons. 

109. Solar Spots. "When the sun is viewed through 
a telescope, furnished with dark-colored glasses, and 
its brilliancy is thereby so much diminished that the 
eye can gaze upon it without injury, dusky spots are 
usually seen upon its surface. Each spot consists of 
two parts, the central portion or nucleus, 1 which is 
the darkest, and around this is a lighter shade called 
the penumbra, 2 usually having the same form as the 
spot, though this is not always the case, as several 
spots are at times included within the same penumbra. 

110. The spots are not permanent, for they are 
sometimes seen bursting out suddenly from the bright 

1. Nucleus^ from the Latin word nucleus, a kernel. 

2. Penumbra, from the Latin pene, almost, and umbra, a shadow : 
i. e., a light shade. 

Apply the rule found to the sun What hare been detected upon the sun's disk ? 



ASTRONOMY. 71 



disk 1 of the sun, and then as rapidly disappearing ; 
one observed by Hevelius appeared and vanished 
within seventeen hours. Their form and size also vary 
from day to day, and even from hour to hour. Some- 
times they are seen to divide and break up into two 
or more separate portions. 

111. Size and Number. The extent and number of 
spots almost exceed belief. M. Schwabe, of Dessau, 
who has examined them with great attention, has dis- 
covered many without the aid of the telescope. In 
June, 1843, one was seen by him with the naked eye 
for the space of a week, which was about 77,000 miles 
in diameter — nearly ten times as broad as the earth. 
Another, mentioned by Sir John Herschel, had a 
diameter of 45,000 miles. This gentleman also ob- 
served at the Cape of Good Hope, toward the close 
of March, 1837, a cluster of spots that covered a space 
3,780,000,000 miles in extent — an area nineteen times 
greater than the entire surface of our globe. 

112. These groups often comprise a great number 
of individual spots. M. Schmidt, of Bonn, counted 
no less than two hundred in a large cluster that he 
examined on the 26th of April, 1826, and in August 
of the preceding year, he detected one hundred and 
eighty in a single group. It is a remarkable fact, that 

1. Disk, the face of the sun, moon, or a planet, as seen from the 
earth, from the Latin word discus, a quoit. 

Describe the spots. Their changes. Within what time has a spot been known 
to appear, pass through its changes, and vanish? What is said respecting their 
size and number 1 Who has examined them with gieat attention 1 What has he 
discovered ? How large a spot did he behold in June, 1843 ? How did it compare 
in breadth with the earth? Give other instances of the magnitude of spots, and 
groups of spots. How many individual spots do the groups sometimes comprise I 
What is regarded as a remarkable fact ? 



72 ASTRONOMY 



although the spots extend over such vast spaces, they 
seldom last more than six weeks. 

113. The number of spots varies much in different 
years. It occasionally happens, that during an entire 
year, spots may be seen upon the sun every clear day, 
while during another year it will be free from them 
for weeks, and even months, together. M. Schwabe, 
who has closely observed the sun for the space of 
twenty-Jive years, has clearly established this fact ; for 
he found that in the years 1 836-7-8 and 9, there was 
not a single day on which the sun was free from spots, 
while in 1843, there were no less than 145 clear days 
when spots could not be seen. 

114. In addition to the spots, the disk of the sun is 
also diversified by branching ridges and streaks, 'more 
luminous than the general surface. These brilliant 
lines are usually found in the vicinity of vast spots 
and clusters, and from their midst the spots them- 
selves not unfrequently break out and spread. 

In Fig. 23, four spots are delineated on the solar 
disk, and in Fig. 24, spots and clusters are shown 
under their various appearances, the nucleus in each 
being represented by the darkest part, and the penum- 
bra by the lightest. 

115. Motion of the Spots. If the sun is watched 
attentively from day to day a spot at its first appear- 
ance will be perceived on the east side of the sun, and 
is then seen to move gradually across the solar disk, 
until at length it disappears on the western side. In 

Does the number of spots vary in different years ? Give instances. How is the 
sun's disk otherwise diversified ? What is stated in respect to these brilliant lines? 
On what side of the sun does a spot first appear? How does it move, and where 
disappear? 



ASTRONOMY. 73 



FIG. 24. 






SOLAR SPOTS. 



this passage it occupies about a fortnight, which is 
the period, of its visibility. After the same lapse of 
time it reappears on the eastern edge. 

This is true with respect to all spots which have 
been observed for this purpose, and whose returns 
have been noted; and the fact that their periods of 
visibility and invisibility are equal, proves that the 
spots are in contact with the sun. For if they were at 
any considerable distance from the body of the sun, 
the time of their visibility would be less than that 
of their invisibility, as can be easily shown by the aid 
of Fig. 25. 

116. In this figure the circle E represents the earth, 
and circle ASB the sun. Now if a spot was not in 
contact with the sun's surface, but moved in the large 
circle CDP, it is obvious that it would be impossible 
for a person at E to see it crossing the sun's surface 
except while it was passing through the a/rc DC. At 
D and C the spot would appear on the edges of the 

What is the period of a spot's visibility and invisibility ? What is proved by 
the equality of these periods ? Show from the figure why the jspots raust Jae in 
contact with the sun. 

4 



74 A ST RON O MF. 



solar disk at B and A, and it would be invisible all 
the time it was passing from C through the rest of the 
circumference of the large circle, round to D again. 

FIG. 25. 




SOLAR SPOTS PERIODS OF VISIBILITY AND INVISIBILITY. 

Now, as the arc CD is much smaller than the other 
part of the circumference of the large circle, to wit, 
CPD, the spot, if it moved uniformly, must be visible 
for a much shorter time than it is invisible, which is 
not the case. 

But if the spot is upon the surface of the sun, it 
will take as long a time for it to move from B to A 
toward E, as from A round to B again, since the 
diameter ASB divides the circumference of circle S 
into two equal parts. The times of visibility and in- 
visibility must consequently now be equal — a conclu*- 
sion in accordance with all observations. 

117. The time that elapses between the appearance 
of a spot at any point on the solar disk, and its reap- 
pearance at the same point, is therefore about four 
weeks (more nearly 27^ days). A spot was observed 

How long a time elapses between the appearance and reappearance of the same 
spot at the same point on the sun ? s surface? How many revolutions has a spot 
been known to make ? 



ASTRONOMY. 75 



in the year 1676, A. D., which made nearly three 
revolutions. 

118. Eotation of the Sun on its Axis. It is by 
means of the solar spots that the rotation of the sun 
on its axis is ascertained, and the period of its rota- 
tion determined. The equality in their times of visi- 
bility and invisibility, and the uniform direction they 
pursue in their passage across the sun's disk, lead to 
the conclusion that the spots have no motion of their 
own ; but, being connected with the body of the sun, 
are all carried forward from west to east by the rota- 
tion of this great orb on its axis. Astronomers have 
differed somewhat in respect to the period of rotation, 
but the best and most careful measurements show 
that the sun rotates once on its axis in the space of 
25d. Yh. 48m. 

119. Physical Nature of the Smsr. Yarious opin- 
ions have been entertained by astronomers respecting 
the constitution of this immense body. La Place 
considered the sun to be a fiery globe of solid mate- 
rials, subject to terrible volcanic action, and that the 
spots are deep cavities, from whence issue at intervals 
vast floods of burning matter, which pour over the sur- 
face of the sun. 

120. Sir "William Herschel regards the sun as a 
dark solid body, surrounded at a considerable distance 
by a stratum of cloudy matter, above which, and near- 
est to us, floats an intensely hot and luminous atmos- 
phere. Whenever these two envelopes, the cloudy 
and the bright, are agitated by any causes existing in 

Are the spots supposed to have a motion of their own ? What is their motion 
the same as? In what time does the sun complete a rotation ? State La Place's 
opinion respecting the constitution of the sun. State Herschel's. 



76 



ASTRONOMY. 



the sun, it frequently happens that they are rent 
asunder, and we perceive through the opening the 
dark body of the sun. Under these circumstances, 
a spot appears. The Hack portion of the sun dis- 
closed, is the nucleus of the spot, and the portions 
of the cloudy stratum illumined by the light from 
the luminous canopy form the penumbra. 

121. In Fig. 26, a section of the sun is delineated 

FIG. 26. 




Spot 

THE SUN HOW CONSTITUTED. 

as it would appear, if Herschel's views are true. In 
this cut the dark circle, S, represents the body of the 
sun, the deeply shaded ring, CC, the cloudy canopy, 
and the outer ring, LO, of a lighter shade, the lu- 
minous stratum. The ruptures in the rings are the 
places of the spots. Looking through any of these 
openings a portion of the dark body of the sun would 

Describe the figure. 



ASTRONOMY 77 



be seen in the centre, forming the nucleus, while the 
shelving edges of the cloudy stratum would constitute 
\hs penumbra. 

122. The theory of Sir "William Herschel affords as 
satisfactory an explanation of the phenomena of the 
sun as any that has been advanced. Spots 45,000, 
and even 77,000 miles across, close up in six weeks. 
The edges must, therefore, approach each other with 
a joint velocity, varying from one thousand to nearly 
two thousand miles a day — a swiftness of motion 
which agrees better with the idea, that the spots are 
ruptures in fluid or gaseous matter, than that they 
are cavities in &firm and solid mass. 

But a late experiment of a French philosopher has 
now proved that the brilliant visible surface of the 
sun cannot consist of either solid or fluid matter in- 
tensely heated, but is composed of inflamed gaseous 
matter — a fact which strongly corroborates Herschel's 
views. 

123. Temperature at the Sun's surface. In gazing, 
then, upon the sun, we look not, according to Her- 
schel's theory, upon the body itself, but on the canopy 
that envelops it; and from the latter flow all the 
light and heat that cheer and invigorate the various 
orbs that revolve around this vast luminary. 

124. The temperature at the sun's visible surface is 
very great, for the hottest fires that rage in the fiercest 
furnaces, but feebly shadow forth the heat that there 
prevails. It can be shown, from reliable calculations, 
that if a given surface, as one square mile, receives, 

Give the reasons why Herschel's theory is most satisfactory. What has been 
lately proved by a French philosopher? From whence do the solar light and 
heat emanate ? What is said respecting the temperature at the sun's surface] 



78 ASTRONOMY. 

at the distance of the earth from the sun, a given 
amount of heat, that the same extent of surface at 
the sun must he. three hundred thousand times hotter. 
Moreover, the brightest flame man can produce, as 
the Drummond light (which is so dazzling that it is 
painful to look upon), appears as a dark spot upon the 
sun when placed between the eye and the solar disk, 
being virtually extinguished by the sun's surpassing 
splendor. 



CHAPTEE II. 

THE MOON. 



125. This beautiful orb is a constant attendant of 
the earth in its circuit about the sun, revolving mean- 
while in the same direction from west to east around 
the earth as its centre. Her influence upon our globe 
is by no means unimportant. Equal in apparent size 
to the sun, her mild splendor dissipates the shades 
of night, while her attractive power sensibly affects 
the motions of the earth, and sways the tides of the 
ocean. 

126. Distance. This orb is the nearest to us of all 
celestial bodies, her average distance being about 
239,000 miles. 

127. Diameter. The moon's diameter, according to 
Prof. Madler, is found to be 2160 miles long — an ex- 
How much hotter is a given surface at the sun than at the earth ? What is said 

respecting the splendor of the solar light? What is the subject of Chapter II. ? 
What is said respecting the motions of the moon, and her influence upon our 
globe I What is said in regard to her distance from the earth ? How far is she 
from the earth ? What is the extent of her diameter ? 



ASTRONOMY. 79 



tent a little greater than one fourth of the earth's 
dia?neter. The relative sizes of the earth and moon 
are shown in Fig. 27, where E represents the earth, 
and M the moon. 

FIG. 27. 




RELATIVE SIZES OF THE MOON AND EARTH. 

128. Moon's Phases. The moon has no light of her 
own, but shines by the reflected light of the sun, the 
hemisphere presented to the sun being illumined with 
his rays, while that which is turned from him is 
shrouded in darkness. The relative positions of the 
sun, moon, and earth, are not always the same, and 
hence arise those periodical fluctuations in the lunar 
light, which are termed the phases 1 of the moon. 

129, From New Moon ~to the First Quarter. At 
new moon, the centres of the sun, moon, and earth, 
are situated in nearly the same straight line, the moon 
being in the middle, at which time she is said to be in 
conjunction. In this position, the unenlightened part 
of the moon is turned towards the earth, and the orb 
is lost to our view. In a short time it advances so far 

1. Phases, from the Greek word phasis, meaning an appearance. 

Does the moon shine by her own light? What is the cause of the periodical 
fluctuations in the lunar light? What name is given to these fluctuations? 
Describe -the phases of the moon from new moon to the first quarter. 



80 ASTRONOMY. 



to the east of the sun as to become visible in the west 
soon after his setting. Its bright portion then ap- 
pears of a crescent form, on that side of the disk 
which is nearest to the sun, while the remaining dark 
part of the disk is just discerned, being faintly illu- 
mined by the earth-light. 1 In this position the convex 
part of the moon's crescent is towards the sun, and 
the line which separates the illumined from the un- 
illuwAned part, called the terminator, is concave. 

130. Each succeeding night the moon is found far- 
ther eastward of the sun, and the bright crescent 
occupies more and more of her disk, the terminator 
gradually growing less curved, until when the moon 
is 90° distant from the sun, half the disk is illumina- 
ted, and the terminator becomes a straight line / the 
moon is then in her first quarter. The extremities 
of the moon's crescent are called cusps, 2 and from the 
time of new moon to the first quarter, the moon is said 
to be horned. 

131. From the First Quarter to Full Moon. As 
the moon advances beyond her first quarter, the ter- 
minator becomes concave toward the sun, and more 
than half the lunar disk is illuminated, when the moon 
is said to be gibbous. 2 At length, in her easterly prog- 

1. Earth-light. Some of the light which falls upon the earth from 
the sun is reflected to the moon, and a portion of this is reflected back 
again from the moon's surface to the earth This is the earth-light. 
The amount thus reflected from the lunar surface must necessarily be 
very small, but it is sufficient to enable us faintly to discern the out- 
lines of the moon. 

% Cusps, from the Latin word cuspis, meaning the point of a spear. 

3. Gibbous, from the Latin word gibbus, meaning swelled out. 

From the first quarter to the full. 



ASTRONOMY. 81 



ress, she reaches her second quarter, and the sun, 
earth, and moon, are again in nearly the same straight 
line, the earth, however, being in the middle. The 
moon is now in opposition, 180° from the sun, rising 
in the east at about sunset / and as her whole enlight- 
ened dish is turned toward the earth, she is now at 
the full. 

132. From Full Mooisr to the Third Quarter. 
After opposition, the enlightened ])&rt of the moon again 
becomes gibbous as she returns toward the sun, and 
she rises later and later every night. When she has 
arrived within 90° of the sun, she is then in her third 
quarter, the terminator is once more a straight line, 
and the bright portion of the orb again fills up one 
half of the disk. 

133. From Third Quarter to New Moon. After 
passing her third quarter the moon resumes her ores- 
cent shape, rising early in the morning before the sun. 
As her time of rising approaches nearer and nearer 
to that of the sun, the glittering crescent contracts in 
breadth, until at length the moon arriving again at 
conjunction, its light entirely disappears. The posi- 
tions of the moon, where she is midway between any 
two adjacent quarters, are termed her octants. 1 

This subject is further illustrated by Fig. 28, where 
S8, SI, and all lines parallel to these, indicate the 
direction in which the sunbeams come, and E repre- 
sents the earth. The circles 1, 2, 3, 4, 5, 6, 7, and 8, 

1. Octant, derived from the Latin word octo, eight — an octant being 
distant from its adjacent quarters one eighth part of the moon's orbit. 

From the full to the third quarter- From the third quarter to new moon. 
What are the octants ? 

4* 



82 



ASTRONOMY 



FIG. 28. 




MOON S PHASES. 



show the places of the moon in her orbit, at conjunc- 
tion (1), the first octant (2), the first quarter (3), the 
second octant (4), at opposition (5), the third octant 
(6), the third quarter (7), and at the fourth octant 
(8) ; while the white portions of the circles l 1 , 2 1 , 3 1 , 
4 1 , 5 1 , 6 1 , 7 1 , and 8 1 , exhibit the phases of the moon 
in all the preceding positions. Thus, when the moon 
is at the first octant (2), the phase corresponding to 
this place is displayed in circle 2 1 , that of the first 
quarter (3), in circle 3 1 ; and so of all the other posi- 
tions. 

134. The points in the moon's orbit, where she is 
one quarter of her orbit distant from the sun, are 
called her quadratures. 1 Fig. 29 exhibits the appear- 

1. Quadratures, derived from the Latin word quadrans* meaning a 
quarter. 



Explain Fig. 28. What are the quadratures ? What does Fig. 29 exhibit ? 



ASTRONOMY. 



83 



FIG. 29. 



B 
o 
o 

25 




84 ASTRONOMY. 



anee presented by the moon in quadrature when seen 
magnified through a telescope. 

135. What the Phases Prove. The phases of the 
moon clearly prove that this body possesses a spheri- 
cal figure^ and is illumined by the sun* for it is only 
a spherical body, which, viewed in the positions we 
have mentioned, can exhibit the phases that the moon 
has displayed through all past time. This point may 
be illustrated in the following maimer: If in the 
evening we place a lamp upon a table, and, taking 
our stand at a distance, cause a person to carry around 
us a small globe, we shall perceive that the illumined 
pari of the globe, in its circuit around us, presents to 
view all the phases of the moon. Being crescent- 
shaped when the globe is nearly between us and the 
lamp y in its first quarter when the lines drawn to it 
from the eye and the lamp make a right angle ; and 
at the full, when it is opposite to the lamp ; and so 
on throughout the entire circuit. 

136. Sidereal Month. Upon observing the moon 
from night to night, we perceive that she has a mo- 
tion among the fixed stars y for if on any particular 
evening she is beheld mear a star, on the next suc- 
ceeding clear evening she will be seen far to the east 
of this star. And thus the moon continues to advance 
from west to east until, in the space of 27 days 7h. 
43m. ll^sec, she makes one entire revolution, occu- 
pying the same position among the stars as she did 
at the commencement of this interval. For this 



What do the phases prove 1 Is the moon stationary or in motion ? How is she 
proved to be in motion 1 In what direction does she move ? How long is she in 
completing a revolution from west to east ? 



ASTRONOMY. 85 



reason the period of time just mentioned is denomi- 
nated a sidereal month. 

137. Synodical or Lunar Month. The time that 
elapses between two consecutive full moons or new 
'moons, is termed a synodical 1 month, and consists of 
29 days 12h. 44m. 3sec. If the earth was stationary 
while the moon revolved around it, the length of the 
synodical month would exactly equal that of the 
sidereal, for the moon in passing from conjunction 
would then be brought round to conjunction again at 
the completion of one revolution. But as it is, while 
the moon is revolving around the earth, the earth is at 
the same time revolving about the sun in the same 
direction, and advances about 29° 6 r , while the moon 
makes one revolution. The moon, therefore, in pass- 
ing from conjunction to conjunction, describes not 
simply 360° or one entire circumference, but about 
389° 6', or nearly one circumference and a twelfth / 
and the time which she occupies in going through 
389° 6', is a synodical month, or 29 days, 12 hours, 
44 minutes, and 3 seconds. 

138. Physical Aspects of the Moon. When the 
moon is full we perceive, even with the naked eye, 
that her disk is not uniformly bright, but that marked 
alternations of light and shade extend over the entire 
surface. By the aid of the telescope, these peculiari- 
ties are more distinctly developed, and chains and 

1. Synodical. Derived from two Greek words, sun, with, or together 
with, and odos, a journey. In union signifying a coming together. 

What is this period termed, and why ? What is meant by a synodical or lunar 
month ? What is its length ? Why longer than a sidereal month ? How many 
degrees does the moon pass through in the period of a synodical month? When 
the moon is full, what appearance does her disk present to the naked eye? What, 
when seen through a telescope ? 



86 ASTRONOMY. 



ranges of mountains are discerned, which the early 
astronomers regarded as seas. These tracts, however, 
are most probably broad plains and precipitous val- 
leys, for there is strong evidence that but little moisture 
exists in the moon, and close observation, moreover, 
shows that these regions are too rugged to be sheets 
of water. 

139. The proof that the surface of the moon is very 
uneven, rising into lofty mountains, and sinking into 
deep valleys, is quite conclusive. In the first place 
the terminator, which, it will be recollected, is the 
line that separates the illumined part of the disk 
from the unillumined, and is in fact the pr of le of the 
moon? s surface, is not a regular unbrohen line. Such 
it would be if the surface of the moon was smooth, 
but it is rough and jagged, as seen in Fig. 29, thus 
revealing the existence of prominences and depressions 
in the lunar surface. 

140. Moreover, near the terminator, long shadows 
fall opposite to the sun within the illumined regions — 
a fact which can only be accounted for by the up- 
rising of mountains which intercept the rays of this 
luminary, just as on the earth lofty peaks and pinna- 
cles cast extended shadows at the rising and setting 
of the sun. 

141. Lastly, beyond the terminator, within the un- 
enlightened parts, bright spots or islands of light are 
seen (Fig. 29), which must be the tops of mountains. 
For since the light of these spots is that of the sun 
reflected from the moon's surface, these luminous points 

What views were entertained by the early astronomers ? Are these tracts seas 
or mountains 1 What facts are stated that prove the surface of the moon to he 
rough, rising into mountains and sinking into valleys ? 



ASTRONOMY. 87 



catch the solar rays only on account of their being 
move elevated than the contiguous regions that are 
veiled in obscurity ; and the farther these spots are 
from the tevminaiov, the higher must the mountains be. 

142. Lunar Mountains. The mountainous regions 
of the moon present a greater diversity of arrange- 
ment than those of the earth. Rugged and pre- 
cipitous ranges are seen, as on our globe, traversing 
the lunar surface in all directions ; but the moon pos- 
sesses, besides, a peculiar mountain formation, termed 
ring mountains, which are detected in every part of 
her visible surface. 

The surface of the moon is more rugged than that 
of the earth ; for though the former is much smaller 
than the latter, yet its mountains nearly equal in alti- 
tude the highest of our own. 

143. Prof. Madler of Prussia, who has studied the 
physical condition of the moon with more success 
than any living astronomer, has constructed, in con- 
nection with Prof. Beer, another Prussian astronomer 
of high reputation, large lunar maps, in which the 
most remarkable spots and regions of the moon are 
laid down with great exactness. Their magnitudes 
have also been ascertained, and their forms delinea- 
ted with the utmost precision. 

144. The heights of no less than 1095 lunar moun- 
tains have been determined by these astronomers, and 
out of twenty measured by Madler, three tower to an 



What facts are adduced in Arts. 139, 140, and 141, -which show that the surface is 
rugged ? What is said respecting the mountainous regions of the moon ? State 
what has been done by Profs. Madler and Beer. How many lunar heights have 
been determined by these astronomers ? What is said respecting the heights of 
twenty measured by Madler ? 



88 ASTRONOMY. 



altitude of more than 20,000 feet, while the rest ex- 
ceed the height of 16,000 feet, or about three miles. 
The names of a few of the loftiest mountains are as 
follows : 

FEET. FEET. 

Newton, 23,800 Casatus, 20,800 
Curtius, 22,200" Posidonius, 19,800 

145. The highest lunar mountain, as we perceive, 
reaches an altitude of nearly 24,000 feet, or about 
four miles and a half, which is nearly the height of 
the loftiest mountains of our globe. If our mountains 
were as much higher than the lunar mountains as the 
earth is larger than the moon, the Himmalehs and 
Andes would soar to an altitude of 16^ miles above 
the level of the ocean. 

145. Lunar Craters. The moon is not only distin- 
guished for lofty mountains, but also for singularly 
formed cavities and craters, which are depressed far 
below the general surface. They are of various sizes, 
and are scattered all over the disk of the moon, being 
however most numerous in the southwestern part. In 
form they are nearly all circular, and are shaped like 
a howl, and from the level bottom of most of the 
larger a conical hill usually rises at the centre/ 

146. Oftentimes the circular walls of these craters 
are entirely below the general surface of the moon, 
but they are usually elevated somewhat above the sur- 
face, forming a ring mountain, whose height on the 
outside is frequently not more than one third or one 

Give the names ana altitudes of the four highest. If the mountains of our globe 
were as much higher than the lunar mountains as the earth is larger than the 
moon, how high would the Andes and Himmalehs soar? What is said respecting 
the lunar craters ? Of their size and forms? What is said in regard to the cir- 
cular walls of these craters ? 



ASTRONOMY. 89 



half of its altitude on the inside, measuring from the 
bottom of the crater to the top of the mountain. 

Twelve craters, according to Schroeter, a distin- 
guished German astronomer, are more than two miles 
deep, and to some of these a depth of over four miles 
is assigned by the same observer. 

147. That these appearances, which are regarded 
as cavities, are such in reality, is evident from the fact, 
that the side nearest the sun is in shadow, while the 
side most remote is illumined by his beams. Just as 
the eastern side of a well is in shadow in the morning 
when the sun shines, while the western side at the 
top is bright with the solar rays. 

148. One of the finest instances of a ring mountain, 
with its enclosed crater, is the spot called Tycho. The 
breadth of the crater is nearly fifty miles, the height 
of the mountain on the inside is about 17,000 feet, 
and on the outside it is not less than 12,000 ; the 
bottom of the crater is, therefore, 5000 feet below the 
general surface of the moon. 

From the centre of the enclosed area a beautiful 
mountain rises to the height of almost one mile. 

149. By the aid of a powerful telescope, Tycho is 
seen as it is delineated in Fig. 30. The ranges of the 
ring mountain are here beheld on the right hand of 
the figure, with their summits bathed in light, while 
their sides opposite to the sun rest in the deepest 
shade. On the left hand, nearest to the sun, the solar 
rays, streaming over the encircling mountain walls 
of the crater, leave half of it in darkness — -the heavy 



How deep are these craters, according to Schroeter 1 State the proofs that these 
spots are really cavities. Describe Tycho. 



90 



ASTRONOMY, 



FIG. 30. 




A RING MOUNTAIN WITH ITS CRATER (TYCHO). 

shadow of the central mountain projecting far into 
the illumined portion. 

150. Many of the craters are of great dimensions, 
the largest being nearly 150 miles in diameter. The 
diameters of the six broadest, as inferred from the 
observations of Prof. Madler, are as -follows : 



MILES. 

149 
115 



MILES. 

143 
113 



MILES. 

127 

96 



And of 148 craters, whose diameters were measured 
by the same astronomer — 

Explain the cut. What is said respecting the magnitude of these craters 1 Give 
the diameters of the six broadest according to Madler's measurements. State what 
is said of the diameters of 148 craters measured by the same astronomer. 



ASTRONOMY. 



91 



2 


were 


between 


i 


and 2 miles wide, 


7 


« 


a 


2 


a 


3 


U 


a 


16 


u 


a 


3 


a 


4 


a 


a 


19 


a 


a 


4 


a 


5 


u 


a 


17 


a 


a 


5 


u 


6 


u 


U 


18 


a 


u 


6 


u 


7 


a 


ii 


11 


a 


a 


7 


u 


8 


u 


cc 


9 


a 


u 


8 


a 


9 


a 


a 


12 


a 


a 


9 


a 


10 


u 


a 



And 36 were aifoue 10 miles across. 

151. Lttnajr Volcanoes. The existence of active 
volcanoes has been announced more than once by 
astronomers. In 1787, Sir William Herschel gave 
notice to the world that he had observed three lunar 
volcanoes in actual operation, two of which were 
either just ready to break out or were nearly extinct, 
while the third was in a state of eruption. The burn- 
ing part of the latter was estimated to be three miles 
in extent, while the adjacent regions were illumined 
with the glare of its fires. Since this period, the 
attentjon of many astronomers has been directed to 
this subject, and their investigations have led to the 
conclusion that the remarkable appearances, which 
were regarded as indicating the existence of volca- 
noes, can be satisfactorily attributed to other causes, 
and the opinion is now prevalent among astronomers, 
that active lunar volcanoes do not now exist. 

152. The aspects of the moon, however, indicate 
that it has been the theatre of intense volcanic action. 

What was the belief of Sir William Herschel in respect to the existence of 
active lunar volcanoes? Have these remarkable appearances been regarded as 
active volcanoes by later astronomers ? What is now the prevalent opinion among 
astronomers ? Are there any indications in the aspects of the moon that active 
volcanoes once existed ? 



92 ASTRONOMY. 



and the ring mountains or craters strikingly reveal 
this fact. "In some of the principal craters," says 
Sir John Hersehel, " decisive marks of volcanic strati- 
fication, arising from successive deposits of ejected 
matter, and evident indications of lava currents 
streaming outward in all directions, may be clearly 
traced with powerful telescopes. In Lord Kosse's 
magnificent reflector, the flat bottom of the crater, 
called Albategnius, is seen strewed with blocks, while 
the exterior of another is all marked over with deep 
gullies radiating toward its centre." 

153. Bulk — Mass — Density. The bulk of the moon 
is equal to ^th part of the bulk of the earth, and her 
mass or quantity of matter is equal to ¥ ^th part of 
that contained in our globe. The moon's density is a 
little more than one half of the density of the earth. 

154. The Moon's Orbit. The orbit of the moon is 
an ellipse, with the earth in one of the foci, and ob- 
servations have shown that it is more elliptical than 
that of the earth. Its inclination to the plane of the 
earth's orbit is 5° 8'. 

155. The Line of the Nodes. The moon, in making 
one revolution about the earth, comes twice into the 
plane of the earth's orbit. These two positions, when 
the centre of the moon is at the same time in the 
plane of the ecliptic, and in that of her own orbit, 
are called the moon's nodes.. 1 A line joining these 
two points, is in loth these planes, and is termed the 

1. From the Latin word nodus, meaning a knot, a connection. 

State the remarks of Sir John Herschel. What is the bulk of the moon compared 
with that of the earth ? Her mass ? Her density 7 What is the figure of the 
moon's orbit ? What is the inclination of the plane of che moon's orbit to that of 
the ecliptic ? What is meant by the moon's nodes ? 



ASTRONOMY. 



93 



line of the nodes. In Fig. 31, EO represents a part 
of the plane of the earth's orbit, MM the moon's 
orbit, A and B the ?noon's nodes, and AB the line of 
the nodes. 

FIG. 31. 




LINE OF THE NODES. 



156. The line of the nodes retrogrades from east to 
west in the direction of the arrows, taking the suc- 
cessive positions AB, A 1 !? 1 , and A 2 B 2 . It makes the 
entire circuit of the ecliptic, in the course of 18 
years 218d. 21h. 22m. 46sec. 

157. The Moon always turns the same Face to- 
wards the Earth. Every observer, whose attention 
has been drawn to the fact, has noticed that the ap- 
pearance of one full moon is almost exactly like that 
of another. There is the same relative arrangements 
of light and shade, and the most remarkable features, 
such as prominent mountains and valleys, are con- 
stantly seen in nearly the same positions on the moon's 
disk. This is indeed true in respect to all the lunar 
phases j for the surface of the moon, as seen at her 

What is meant by the line of the nodes? Explain the figure. Are the nodes 
fixed in space 1 Explain from figure. In -what direction does the line of the nodes 
appear to revolve 1 In what period does the line of the nodes make a complete 
revolution ? How does the appearance of the moon at any phase, during any one 
month, compare with her appearance at the same phase during any other month ? 



94 ASTRONOMY 



first quarter, is that which has been seen at every 
first quarter since the creation, and the same which 
will be seen at the same phase, as long as the sun, 
moon, and earth, endure. 

158. This singular phenomenon can be explained 
only on the supposition, that the moon rotates on her 
axis in about the same time that she completes a 
sidereal revolution around the earth y for if she did 
not thus rotate, we should see the greater part of her 
surface in the course of a month, which is not the case. 

159. This point may be thus illustrated : "We will 
suppose a person standing in the middle of a floor, 
and another walking around him in a circle, holding 
up at a level with his eye a globe, of which the sur- 
face of one hemisphere is painted black, and that of 
the other white. The first person represents a spec- 
tator upon the earth ; the circle in which the second 
walks the orbit of the moon, the globe is the moon y 
and the white surface the side that she constantly 
presents towards the earth. Now it is manifest, that 
if the second person, walking round the circle, wishes 
the spectator at the centre to see nothing but the white 
surface of the globe, as he performs his circuit, he 
must turn the globe round on its vertical axis at 
exactly the same angular rate that he himself is 
moving in the circle. Thus, when he has moved 
through one quarter of the circle, the globe must 
have turned one quarter of a circle; when he has 
traversed one half of the circle, the globe must have 
turned half round, and so on through the entire 
circle. 

How can this phenomenon be explained ? Give the illustration. 



ASTRONOMY. 95 



160. Length of the Lunar Day. The moon, as we 
have seen, rotates on her axis in the same period that 
she completes a sidereal revolution about the earth, 
moving forward in the mean while with the latter 
around the sun through an arc of nearly 27°. Owing 
to these two motions, the average length of the day 
at the moon, reckoning by solar time, is equal to the 
length of a synodieal month, that is, to about 29| of 
our days (29 days 12h. 44m. 2.9sec.) The mean lengths 
of daylight and night are therefore respectively equal 
to nearly 15 of our entire days of 24 hours duration. 

161. The Appearance of the Earth as seen from 
the Moon. To the inhabitants of the moon (if any 
there are), our earth is seen as a moon of immense 
size, its apparent surface being sixteen times greater 
than that of the sun as he appears to us. For this 
reason a vast amount of light must be reflected from 
our globe to the moon, and all the varied lunar phases 
which we behold would be exhibited by the earth to 
a lunar spectator with a wonderful radiance and dis- 
tinctness, but in an inverse order. Thus, when it is 
new moon to us it would be full earth to an observer 
on the moon, and whenfidl moon here, new earth there. 

162. Another remarkable difference also exists. The 
moon is seen by us occupying various positions in the 
heavens, as she displays her successive phases ; but 
the earth would appear to an inhabitant of the moon 
to be fixed vn the heavens during all her periodical 
fluctuations of light. The cause of this singular phe- 

What is the mean length of the lunar day, measured by our days? How would 
our earth appear to an inhabitant of the moon ? In what order would the phases 
of the earth be exhibited ? Would the earth have any apparent motion as seen 
from the moon ? 



96 ASTRONOMY. 



nomenon is easily explained. The moon turns on her 
axis from west to east just as the earth does, but an 
inhabitant of the moon would be as unconscious of 
its rotation as we are of the rotation of the earth. 
Accordingly, as with us, the sun and the other fixed 
heavenly bodies would appear to him to be moving 
from east to west, at the same rate that his own orb 
rotates on its axis. Such would be the apparent mo- 
tion of the earth to a spectator upon the moon if the 
earth was actually stationary / but this is not the case, 
for our globe advances from west to east in her orbit, 
just as rapidly as the rotation of the moon tends to 
give it an apparent retrograde motion from east to 
west. 1 The earth, therefore, apparently moving in 
one direction exactly as fast as it actually moves in 
the opposite direction, consequently seems to an in- 
habitant of the moon to stand still in the heavens. 

163. These phenomena would only be seen by a 
spectator on the side of the moon nearest to us, for to 
those inhabiting the remote hemisphere the earth 
would newer come into view. Their long nights of 
nearly 15 days' duration would, therefore, be extremely 
dark, since the brightest heavenly bodies, whose light 
could dissipate the gloom, are Mars and Jupiter, which 
would afford no more illumination to the inhabitants 
of the moon than they do to us. 

1. The moon would present the same phenomenon to us if she com- 
pleted a revolution in her orbit in a sidereal day, for she would then 
actually move as fast from west to east as she would apparently move 
from east to west, on account of the rotation of the earth. Under these 
circumstances, she would seem not to move at all. 

Give the explanation. Could these phenomena he seen from every point of the 
moon's surface? Why not? What is said respecting the nights that prevail 
throughout that hemisphere of the moon which is turned from us ? 



ASTRONOMY. 97 



CHAPTER III. 

ECLIPSES OF THE SUN AND MOON. 

164. The eclipses of the sun and*moon are among 
the most grand and sublime of the phenomena of the 
heavens. In all ages of the world, they have been 
viewed by the ignorant with wonder and awe ; while 
to the man of science they have ever been subjects of 
deep interest and profound study. 

165. Lunak Eclipses. An eclipse of the moon is the 
partial or total obscuration of her light when she 
passes into the shadow of the earth. The sun, earth, 
and moon, are then in nearly the same straight line 
with the earth between the other two bodies. If the 
moon were self-luminous, like the sun, a lunar eclipse 
could never occur / but, shining as she does by reflec- 
tion from the sun, the interposition of the solid body 
of the earth cuts off the solar light, and the portions 
of the moon that enter the earth's shadow appear dark 
to our view. A lunar eclipse can never happen except 
when the moon is full, for it is only at this time that 
the earth is between the sun and moon, and its shadow 
is extended in the direction of the latter. 



Of what does Chapter III. treat ? What is s;aid respecting the eclipses of the 
sun and moon ? What is an eclipse of the moon ? When it occurs, what are the 
relative positions of the sun, moon, and earth? If the moon was self-luminous, 
would there be any lunar eclipses ? In what phase must the moon "be when a 
lunar eclipse happens? 

5 



98 ASTRONOMY 



If the plane of the moon's orbit coincided exactly 
with the plane of the ecliptic, she would pass through 
the earth's shadow at every revolution, and a lunar 
eclipse would take place at every full moon. But as 
the former is inclined to the latter at an angle of 
about 5° (Art. 154), the shadow of the earth may at 
one time pass above the full moon, and at another 
below it. The full moon must, therefore, take place 
within a certain distance of one of her nodes?- that is, 
near the plane of the ecliptic, to make it possible for 
an eclipse 2 to occur. 

166. When the moon, at the full, has her centre 
exactly at her node, it is in the same straight line with 
the centres of the sun and earth, and she is placed 
centrally in the shadow of the earth. But it is not 
necessary that the moon should be precisely in this 
position in order that an eclipse may happen ; for 
since she possesses an apparent breadth of about 30', 
and the shadow of the earth extends on each side of 
the node, her disk may be obscured when she is within 
a short distance of this point. 

167. Of the Eauth's Shadow. In Fig. 32, where S 

1. It will be remembered that the moon's nodes are those points in 
her orbit where the latter intersects the plane of the ecliptic. They 
are consequently at once in the plane of the moon's orbit, and in that 
of the earth's. i 

2. Eclipses are so called from the fact here stated, viz., that they 
occur in or near the plane of the ecliptic. 



If the plane of the ecliptic and that of the moon's orbit coincided, how often 
would lunar eclipses occur? Why do they not now take place every month? 
Near what point must the full moon be to make it possible for an eclipse to happen ? 
Explain why it is not necessary for the moon to be exactly at one of her nodes for 
this phenomenon to occur ? 





ASTRONOMY. 


99 




FIG, 32. 




A 


M 


^.^u 




^^^cz^-----^ ___^ B^^ 


-•*■"" 


f S' 


\ "~ ---._._ " ~t^cl1/' fev "•"^Ig 


s^^^= 




/ --"""' ~ - v J<y/y— — .p 


, : -"*- B 




^^— "- " — L --==# 





p ' 


ECLIPSE OF THE MOON. 


^^W 



represents the sun, and E the earth ; the dark portion 
DBL is the earth's shadow. Its length from E to B 
is on an average 860,000 miles. On each side of the 
shadow there exists, to a certain limit, a space where 
there is & partial shadow , or penumbra. 1 Outside of 
this space the moon is illumined by the full orb of 
the sun, but as she enters the penumbra the dark 
body of the earth begins to interpose itself, and cuts 
off a portion of the sun's light. As she continues to 
approach the shadow, more and more light is inter- 
cepted, and at the moment the earth totally hides the 
sun from any part of the moon, that part at the same 
instant passes the inner limit of the penumbra and 
enters the shadow. 

168. The space occupied by the penumbra is deter- 
mined as follows : Referring to Fig. 32, and sup- 
posing the lines ALW and PDU to be drawn, touching 
the earth at the points D and L, the penumbra is 
found on each side of the shadow, bounded by the 
lines UD, DB, and BL, LW. QM represents the path 
of the moon, and the several small circles on the line 

1. See Note 2, Art. 109. 

What is- the length of the earth's shadow in miles ? What is the penumbra ? 



100 ASTRONOMY. 



QM are different positions of the moon at and near 
the time of an eclipse. 

169. When the moon is entirely obscured, the eclipse 
is called total / when only & portion of the disk is con- 
cealed, partial ; and when the disk just touches the 
edge of the shadow, the phenomenon is termed an 
appulse. 

170. Red Light of the Disk. During a lunar 
eclipse the darkened surface of the moon is illumined 
by a reddish light, a phenomenon resulting from the 
refraction of the solar rays by the eartlus atmosphere. 
For the solar beams entering our atmosphere are 
refracted towards the earth, $nd being thus bent into 
the shadow, pass onwards and strike the moon. Being 
thence reflected to us, they are still sufficiently bright 
to render her surface, even in shadow, distinctly 
visible. The color of the light is owing to the same 
cause that gives rise to the ruddy tints of sunset 
clouds; the white light of the sun, in struggling 
through the atmosphere, loses its feebler rays, 1 while 
the red, which possesses the greatest power to over- 
come any resistance it encounters, emerges, and im- 
parts its own hue to the objects upon which it falls. 

This reddish light is of sufficient intensity to enable 

1. When a sunbeam is refracted, the seven colors of which it is com- 
posed, to wit, red, orange, yellow, green, blue, indigo, and violet, are 
turned out of the course of the original beam. The red deviating the 
least and the violet the most. The red is therefore least affected by the 
resistance it meets with. 



When is an eclipse total, when partial, and when does an appulse occur ? 
What phenomenon occurs during a lunar eclipse? How is it caused? To what 
is the color owing? 



ASTRONOMY. 101 



observers to detect the obscure regions and spots on 
the lunar disk. The following facts are stated by 
Hind. During an eclipse of the moon that occurred 
on the 23d of July, 1823, M. Gambart saw all the 
lunar spots distinctly revealed. In an eclipse that 
happened on the 26th of December, 1833, Sir John 
Herschel observed, that the moon was clearly visible 
to the naked eye, when completely immersed in the 
earth's shadow ; gleaming with a swarthy copper hue, 
which changed to bluish green at the edges, as the 
eclipse passed away. Similar phenomena were noted 
during the total lunar eclipse of March 8th, 1848. 

The spots on the surface, even at the middle of the 
eclipse, were distinctly seen by many observers, and 
the general color of the moon was a full glowing red. 
So clearly did the lunar disk stand forth to view, that 
many of the observers doubted if there was any eclipse 
at all. - 

171. Earliest observations of Lunar Eclipses. 
Observations were made on lunar eclipses at Baby- 
lon, by the Chaldeans, in the years 719 and 720 B.C. 
They relate to three ec^ses, and are the earliest ob- 
servations of this kind, in the annals of science. The 
first eclipse occurred on the 19th of March, 720 B.C., 
and was total at Babylon. The second happened on 
the 8th of March, 719 B.C., and the third, on the 1st 
of September in the same year ; both were partial 
eclipses. 

172. Eclipses of the Sun. An eclipse of the sun 
takes place when the moon in her revolution about 

What can be discerned on the disk of the moon by means of this light ? Detail 
the facts mentioned by Hind? Give an account of the earliest observations of 
lunar eclipses. 



102 ASTRONOMY. 



the earth, comes between the earth and the sun, and 
casts her shadow upon the former ; concealing from 
our view, by her interposition, either a part or the 
whole of the bright disk of the sun. A solar eclipse 
can therefore only occur at the time of new moon or 
conjunction ; and, as in .the case of lunar eclipses, it 
would happen every revolution, if the plane of the 
ecliptic coincided with that of the moon's orbit. But 
this is not the fact, and a solar eclipse can therefore 
only take place when at new moon the lunar orb is at 
or near one of her nodes. The greatest possible dis- 
tance of the moon from the node, at which a solar 
eclipse can occur, is 18° 36'. 

173. Form of the Eclipse. A solar eclipse may be 
partial, total, or annular. It is partial when only a 
portion of the dark body of the moon interposes be- 
tween the sun and a spectator upon the earth ; total, 
when the apparent diameter of the moon exceeds that 
of the sun, and the former body passes nearly cen- 
trally across the solar disk ; annular, when the moon 
passes in like manner nearly centrally before the sun, 
but her apparent diameter is less than the solar / the 
entire body of the sun being then obscured with the 
exception of a brilliant ring, around the borders of the 
moon. When in this case the centres of the sun, 
moon, and earth, are exactly in the same straight line, 
the eclipse is termed annular and central, and the 
bright ring possesses a uniform breadth. 

What is the cause of a solar eclipse? At what phase of the moon can it only 
occur? Why not at every new moon? Where must the new moon occur? What 
is the greatest possible distajnce from the node that a solar eclipse can take place ? 
What is stated respecting the form of a solar eclipse ? When is impartial? When 
total? When annular? When annular and central? 



ASTRONOMY. 103 



174. Shadow of the Moon. The distance of the 
moon from the sun is subject to variation, and this 
circumstance affects the length of the moorfs shadow. 
The farther this orb is from the sun, the longer will 
be her shadow, and the nearer the shorter. 

175. The average length of the moon's shadow is 
found to be about equal to her mean distance from 
the earth. It will accordingly, for the reasons above 
assigned, at times fall short of the earth, while at 
others it will be so much extended, that a shadow of 
considerable breadth passes over the surface of the 
globe. . 

176. When the shadow does not reach the earth, it 
is manifest that no total eclipse can occur; though 
the sun, moon, and earth, may be so situated in every 
other respect as to tend to cause this phenomenon. 
"When it does reach the earth, the space that it covers 
on the surface of the latter, will depend upon the po- 
sition of the end of the shadow in reference to the sur- 
face of the earth. If the end of the shadow just 
touches the earth, there will be an eclipse only at the 
place where it touches. But if the point where the 
shadow would terminate, if the earth did not inter- 
pose, is situated, as at F in Fig. 33, far on the other 
side of the earth, then the eclipse will be visible 
throughout a region of considerable extent. The 
largest extent of surface on the earth, covered at once 
by the shadow of the moon, is about 180 miles. 

State the cause of the variations in the length of the moon's shadow. When is 
it shortest ? When longest ? To what is the average length of the moon's shadow 
nearly equal 1 What happens if it is less ox greater than the mean length 1 When 
will no total eclipse occur ? Upon what does the extent of terrestial surface covered 
by the shadow depend? Give the two illustrations. What is the greatest extent 
of surface obscured by the shadow ? 



104 ASTRONOMY. 



177. The lunar shadow, like that of the earth, has 
also its penumbra, which partially obscures our globe. 
The greatest breadth of terrestrial surface enclosed by 
the penumbra is nearly 5000 miles. 




178. In Fig. 33, this subject is illustrated. S here 
represents the sun, M the moon, and E the earth. 
The form of the shadow is defined by the line CF 
and DF ; a portion of the shadow is, however, cut off 
by the interposition of the earth. The breadth of the 
shadow on the earth is represented by the distance 
from O to P, and the breadth of the penumbra on 
each side of the shadow, by the curved lines GO, and 
PH. 

179. Total Eclipse of the Stot. We have re- 
marked that eclipses of the sun and moon are among 
the grandest phenomena in nature ; but no form of 
eclipse is so impressively sublime as a total eclipse of 
the sun. The gradual withdrawal of the solar light, 
and at length its total extinction ; the oppressive and 
unnatural gloom that overspreads the earth, so dif- 
ferent from the obscurity of night, and the appearance 
of the stars, at such an unusual time, all impress the 
mind with a deep solemnity. It is not surprising that 



State what is said respecting the penumbra and its breadth. Illustrate from 
Fig. 33. What is said in respect to a total eclipse of the sun ! 



ASTRONOMY. 105 



a spectacle of this kind has ever filled barbarous, and 
even civilized nations, with astonishment and dread, 
as though they were on the brink of some awful 
calamity. 1 But eclipses, whether total or otherwise, 
are the source of one of the noblest triumphs of science; 
for astronomers are now so well acquainted with the 
laws that regulate the motions of the heavenly bodies, 
that the very minute of an eclipse can be predicted cen- 
turies before it occurs, and the dates of events which 
happened thousands of years ago, can be unerringly 
fixed, by retrograde calculations of these phenomena. 2 
180. During a total eclipse of the sun, many sin- 
gular appearances are usually observed. Soon after 
the eclipse has commenced, and as it gradually ad- 

1. A total eclipse of the sun occurred during the war between the 
Medes and Lydians, related by Herodotus. In the midst of a battle, the 
sun was blotted out from the sight of the contending armies, and so 
great was their terror at such a strange event, that they threw down 
the weapons, and made a peace upon the spot. This eclipse is said to 
have been predicted by Thales. 

2. When Agathocles, the tyrant of Syracuse, invaded Africa, for the 
purpose of attacking the Carthaginians in their own country, a total 
eclipse of the sun occurred at the time the expedition was setting sail. 
This circumstance disheartened the soldiers, but Agathocles revived 
their courage by representing that this event portended the defeat and 
ruin of their enemies. This eclipse occurred, according to retrograde 
calculations, on the 15th of August, 310 B.C. An eclipse of the sun 
also happened at the very time Xerxes set out from Sardis, to invade 
Greece. The eclipse proves that this historical event occurred on the 
19th of April, 481 B.C. A lunar eclipse which happened on the 21st 
of September, 331 B.C., fixes the date of the battle of Arbela, in which 
Alexander triumphed over Darius, king of Persia. The eclipse oc- 
curred eleven days before the victory. 

How have these phenomena been regarded by barbarous, and even civilized na- 
tions ? What have they proved to astronomers ? 



106 A S T R O N O M Y. 



vances, jets of light are sometimes seen flashing over 
the lunar disk ; and as the total obscuration ap- 
proaches, the bright portion of the sun changes color 
by degrees, either becoming fainter than before, or 
else assuming a reddish tinge. 

When the sun is completely hidden, a beautiful 
ring or corona} of light appears around the dark body 
of the moon, like the crown of light or glory with 
which painters surround the heads of saints. In the 
eclipse of 1842, one observer describes it as a ring of 
peach-colored light, another as white, and a third as 
beaming with a yellowish hue. Its breadth likewise 
does not always appear to be the same ; for in the 
eclipse just mentioned, while some observers esti- 
mated the width at one eighth of the moon's diameter, 
others saw radiations of the corona eight times as long 
as the moon's diameter. The breadth of the corona 
noticed by Mr. Bond, during the eclipse of July 28, 
1851, was about one half of the sun's diameter. 

181. But the most brilliant phenomena remain to 
be described. When the sun is completely concealed, 
and the corona is displayed, rose-colored flames appear 
to dart out from the edge of the moon, emanating from 
the bright ground of the cotona, and so distinct that 
they are frequently visible without the aid of the 
telescope. They vary from two to four in number, 
and though mainly of a rose color, yet they are seen 
tinged with lilac, greenish blue, and purple. During 
the eclipse of July 28th, 1851, Prof. Bond, of Cam- 

1. Corona, a Latin word signifying a crown. 

Describe the various appearances that are beheld during a total eclipse of the 
sun. 



A ST R ON O M Y. 107 



bridge, noticed these beautiful rose-colored flames, 
two of which were connected by an arch of light, re- 
sembling a rainbow. 

FIG. 34. 




TOTAL ECLIPSE OF THE SUN, AS SEEN BY MR. J. R. HIND, NEAR 
ENGELHOLM, IN SWEDEN, JULY 28, 1851. 

182. Fig. 34 represents this eclipse as seen by Mr. 
J. R. Hind, ki Sweden. The eclipsed sun is here 
seen surrounded by a corona, the whiter portions of 
which near the. dark circle indicate the positions of 
the^s of flame and the arch of light. 

Wkt appearances were observed by Mr. Bond, during the eclipse of July 28tb, 
1851 ? Describe Fis\ 34. 



108 ASTRONOMY. 



183. Solar and Lunar Eclipses — Points of Dif- 
ference. When a lunar eclipse occurs, it can be seen 
from every part of that side of the earth, which is 
turned towards the moon. For this hemisphere is 
necessarily in the earth's shadow, and a spectator here 
situated beholds the moon eclipsed when she enters 
the shadow. 

184. In the case of a solar eclipse, the shadow of 
the moon passes across the earth, and an eclipse can 
only occur in the path of the moon's shadow. Every 
part of the terrestrial hemisphere turned toward the 
sun, will not, therefore, be eclipsed, but only those por- 
tions that are traversed by the lunar shadow. These 
differences in respect to lunar and solar eclipses, 
arise from the different positions of the observer 
in the two cases. During a lunar eclipse he is on the 
body that forms the shadow ; during a solar eclipse 
he is on the body that receives the shadow. 

185. Frequency of Eclipses. Seven is the greatest 
number of eclipses that can occur in the course of a 
year, and two the least. If seven take place, Jive may 
be solar and two lunar, or three may be eclipses of 
the sun and four of the moon. Six eclipses in a year 
is an unusual number, four the average, and two the 
least / in the last case the eclipses will be solar. 



State in what respects solar and lunar eclipses differ. Flow do these differences 
arise? What is the greatest number of eclipses that can orcur in a year? What 
the least? If seven take place, what will be the number of solar eclipses, and what 
the numlier of lunar? What is an unusual number in a year? What the average? 
What the least number ? If only two occur, are they solar or lunar? 



ASTRONOMY. 109 



CHAPTEE V. 

THE PLANETS. 

186. The planets are those heavenly bodies that 
revolve directly about the sun, 1 from west to east, 
and shine by its reflected light. They have received 

♦this appellation, as we have stated (Art. 2, Note 1), 
from the fact that they are seen moving among the 
fixed stars, and are constantly changing their places 
in the heavens. 

187. The names of the different planets have already 
been given (Art, 6). Mercury, Yenus, Mars, Jupi- 
ter, and Saturn, have been known from the earliest 
ages, for they are visible to the naked eye, and, all 
but Mercury, conspicuously so. The rest of the 
planets, 44 in number, excluding the Earth, are 
recent discoveries, all of these having been found 
since the year 1780, and 38 of them within the last 
12 years. 

Many of the planets are attended by moons, like 
the earth. The earth, as we know, has one moon — 

1. The planetary bodies that revolve directly about the sum are called 
primary planets. Moons are termed secondary planets. The body 
about which another directly revolves is denominated its primary ; 
thus : the sun is the primary of the earth, and the earth the primary of 
the moon. 



What are planets ? Why are they so called ? Which have been known from a 
high antiquity 1 Kow many have been discovered since the year 1780? How 
many within the last 12 years ? How many planets have moons, and what is the 
number of moons that each of these respective] v have 1 



HO ASTRONOMY. 

Jupiter four, Saturn eight, Uranus six, and Neptune 
one. Up to the present time, the known number of 
planets, including the Earth, is 50, and of moons 20. 

There doubtless exist other planetary bodies in our 
system yet undiscovered, if we can infer anything 
from the harvest of planets that has lately rewarded 
the searching labors of zealous astronomers. 

188. The planets are named after the personages 
of the classic mythology, and are distinguished by 
appropriate symbols. Thus, the symbol of Venus, 
the goddess of beauty, is a mirror / that of Mars, the 
god of w r ar, a spear and buckler. The sign of Vesta, 
the goddess of fire, is an altar / that of Ceres, the 
divinity that presides over harvest, a sickle; while 
Neptune, the god of the ocean, has for his symbol a 
trident; and so of others. The symbols of the as- 
teroids are, for the most part, figures, which repre- 
sent the order in which they were discovered. 

189. Universal Attraction. It is the attractive 
force of the sun that causes the planets and every 
body of the solar system to revolve about this mighty 
orb. Indeed it is an universal law, that all bodies 
mutually attract each other in the direct ratio of 
their quantities of matter, and in the inverse ratio of 
the squares of their distances from each other. For 
example, if the moon contained twice as much mat- 
ter as it now does, it would, at its present distance, 
attract the earth twice as much ; but if it was half its 
present distance from the earth, though it contained 



What is the known number of planets at the present time ? What the num- 
ber of moons ? Are there reasons for believing that other planets will be discovered ? 
What is said of universal attraction? 



ASTRONOMY. HI 



no more matter than it now does, its attraction would 
be increased four times. 

190. Kepler's Laws. The great astronomer Kepler, 
who lived about 250 years ago, discovered three 
great laws of planetary motion, which, from their 
importance, are termed the laws of Kepler. They 
are enunciated as follows : 

First Law. — The planets move in ellipses around 
the sun, which occupies a focus common to all these 
ellipses. 

Second Law. — The radius-vector describes areas 
proportional to the times. x 

Third Law. — The squares of the periodic 2 times of 
the planets are proportional to the cubes 2 of their 
average distances from the sun. 

191. The respective distances of the planets from 
the sun, beginning with the nearest, are presented in 
the following table. The distances of a few "of the 
latest discovered asteroids have not yet been ascer- 
tained. 

1. If we were to imagine the sun to be joined to the earth by a rod, 
the rod would be the radius-vector; and as the earth carried one end 
of the rod around, the other being fastened to the sun, the areas or 
spaces swept over by the rod in equal times, as days or weeks, would 
be always equal. This is true of all the planets. 

2. Periodic time is the time occupied by a planet in performing one 
revolution about the sun. Thus, one year is the periodic time of the 
earth. 

3. A cube is the quantity resulting from multiplying a quantity into 
itself twice ; thus : 8 is the cube of 2, because 2X2X2 equals 8. 



State the laws of Kepler. Enumerate the planets, and give the distances. 



112 



ASTRONOMY. 



MILES. 

MERCURY. 30,890,000 

VENUS, 68,770,000 

EARTH 95,298.260 

MARS, 145,205,000 



ASTEROIDS. 

FLORA, 209,131,670 

H ARMONIA, 215,441 ,000 

ISIS, 217,531,000 

MELPOMENE 217,890,100 

VICTORIA, or CLIO, 221,794,600 

EUTERPE, * 223,046,035 

URANIA, . . . .\ 224,041,065 

VE STA, 224,253,105 

POLYMNIA, - 225,985,050 

METIS, 226,700,875 

IRIS 226,7 18,830 

PHOCE A, 228,063,935 

MASSALIA, 228,139,650 

HEBE, 230,327,785 

LUTETIA, 231.240,070 

FORTUN A, 232,187,030 

PARTHENOPE 232,569,215 

THETIS, 235,971,260 

AMPHITRITE, 241,898,215 

EGERIA, 244,804,550 

ASTRE A, 244,853, 190 



MILES. 

IRENE 245,536,620 

POMONA 245,580,320 

PROSERPINE, 245,841,190 

FIDES 247,399,000 

EUNOMIA 251 ,123.950 

THALIA, 251,286,780 

JUNO,. 253,518,005 

CIRCE, 253,916,000 

CERES, 262,747,675 

L^ETITIA 262,969,500 

PALLAS, 263,105,635 

BEL'LON A, 264,168,875 

CALLIOPE, 276,612,450 

PSYCHE, 278,630,345 

THEMIS 298,727,120 

HYGEIA, 299,191,480 

EUPHROSYNE, 303,267,265 



LEUCOTHEA, 
ATALANTA,. 

LEDA, 

DAPHNE,.... 



JUPITER, 495,817,000 

SATURN, 909,028,000 

HERSCHEL, or UR AN US,1, 828,071, 000 
NEPTUNE, 2,862,457,000 



192. The average velocity of electricity through 
the telegraphic wires is about 16,000 miles per Second. 
If, therefore, for example, London was united to New- 
York by a telegraphic line, news could be sent from 
one city to the other in about one fifth of a second. 
Now, supposing the sun was connected with the 



Take the velocity of the electric current as the unit of measurement, and give 
the different estimates of the planetary distances with this unit. 



ASTRONOMY. 113 



planets by telegraphic lines, then the time it would 

take to transmit a message from the 

Sun to the Earth, would be 1A. 39' 

to Jupiter, " 8A. 36' 

to Saturn, " 15A. 47' 

to Herschel, " Id. 7A. 44' 
to Neptune, " 2d, 1A. 42' 

193. Apparent Size. The apparent magnitude of a 
body is inversely proportioned to its distance ; that is, 
if at a certain distance it appears of a certain size, 
when ten times nearer it will appear ten times larger, 
and if five times farther off, five times smaller, and 
so on. 

It follows, therefore, that the sun will appear of 
various sizes at the -different planets. The relative 
apparent magnitudes of this body, as it would be 
seen from the eight principal planets, are shown in 
Fig. 35. 

194. Kepler's Law of Distances. From the third 
law of Kepler, viz., that the squares of the periodic 
times of the planets are as the cubes of their mean dis- 
tances from the sun, the unknown mean distance of a 
planet can be found, when its periodic time is ascer- 
tained, together with the distance and periodic time 
of another planet. 

Thus the periodic time of Mars having been ascer- 
tained by observation to be 687 days, and the distance 
of the earth from the sun and her periodic time being 
known, the mean distance of the former can be found 



What is said respecting the apparent size of the sun as viewed from the different 
planets? When can the distance#of a planet be found by Kepler's third law? 
Give an instance. 



114 ASTRONOMY 



by the following proportion, viz. : the square of the 
earth'' s periodic time is to the square of Mars' periodic 
time, as the cube of the earth's distance is to the cube 
of Mars' distance. 1 

In this way the periodic time of a planet can also 
be found, when its distance is known, and also the 
distance and periodic time of another planet. 

195. The laws of Kepler are alike applicable to 
moons and planets. The mean distances of the former 
from the planets about which they revolve can, there- 
fore, be determined, as in the case of planets, by the 
law just mentioned. 

196. Division of the Planets. The planets are 
usually divided into two classes. First, the Inferior, 
whose orbits are within that of the earth : Mercury 
and Venus constitute this class. Secondly, the Supe- 
rior, whose orbits inclose the earth's orbit : within 
this division are comprised all the planets from Mars 
to Neptune inclusive. 

197. Inferior Planets. The two planets, Mercury 
and Venus, are known to have their orbits within 
that of the earth : First, because they are never seen 
by us, like the other planets, in a part of the heavens 
opposite to that which the sun occupies, which would 

1. This proportion, expressed in figures, is as follows : (365.256 
[days] X 365,256) : (687 X 687) : : (95,298,000 [miles] X 95,298,000 
X 95,298,000) : (145,210,000 [miles] X 145,210,000 X 145,210,000). 
The last term is the cube of Mars' mean distance from the sun. The 
distance is, therefore, 145,210,000 miles. 

Can the periodic time of a planet be found by this rule ? Is this law applicable 
to moons? Into how many classes are the planets divided? What are they? 
What is meant by an inferior^ what by a superior planet ? How do we know that 
the orbits of Mercury and Venus are within that of the earth ? 



ASTRONOMY. 



115 



•FIG. 35. 

APPARENT SIZE OF THE SUN, RELATIVE SIZES OF THE EIGHT 
AS VIEWED PRINCIPAL PLANETS. 




116 ASTRONOMY. 



be the case if they included the earth within the 
circuit of their respective orbits. 

198. Secondly : if viewed with a telescope, they 
present phases like the moon, being crescent-shaped, 
when situated between the earth and the sun, and 
full when the sun is between them and the earth, 
and in other positions exhibiting every variety of 
phase between these two extremes — phenomena 
which can be accounted for only on the supposition 
that these planets receive light from the sun, and 
move around it at a nearer distance than the earth. 

199. Thirdly: because these bodies, at certain 
times, are seen between the earth and sun, appearing 
as dark spots on his disk, as they cross from one side 
to the other. Such an appearance is termed a transit 
When either of these planets is between the earth and 
the sun, it is said to be in inferior conjunction / wh§n 
the sun is between it cmd the earth, it is in superior 
conjunction. 

Mercury. 5 

200. This planet is the nearest to the sun of any that 
have been discovered. Its greatest angular distance 
from this luminary never reaches 29°. For this 
reason, it can only be discerned in the gloom of twi- 
light, either at morning or evening, according as it is 
to the east or west of the sun. Even under the most 
favorable circumstances, it does not appear conspicu- 



What is the interposition of a planet between the earth and the disk of the sun 
called ? When are these planets respectively in their inferior and superior con- 
junctions ? What is said respecting the proximity of Mercury to the sun 1 What 
is the extent of its greatest angu.ar distance from this orb ? When can this planet 
be seen? When is it most conspicuous? What is its appearance? What is its 
distance from the sun ? 



ASTRONOMY. 117 



ous to the unaided eye, but shines like a small star 
beaming with a pale red light. Its average distance 
from the sun is 36,890,000 miles. Its orbit is very 
elliptical, and the plane of its orbit is inclined to that 
of the ecliptic about 7°. From measurements taken 
with the utmost accuracy within the last few years, the 
diameter of this planet is estimated to be 2950 miles, 
and it revolves about the sun in nearly 3 months, or 
more exactly 87d. 23h. 15m. 44sec. 

201. Rotation on its Axis. The powerful illumi- 
nation to which this planet is subjected on account of 
its proximity to the sun, has thrown a degree of uncer- 
tainty upon all investigations respecting its physical 
characteristics, and the number of reliable observa- 
tions upon it is therefore few. Sir William Herschel, 
with all his ability and skill, obtained no conclusive 
proof of the existence of spots upon the surface of the 
planet, which would have enabled him to determine 
the time of its rotation on its axis. Schroeter appears, 
however, to have met with better success. In the 
early part of this century he subjected Mercury to a 
most careful scrutiny, and obtained, as he believed, 
decisive evidence of the existence of mountains , rising 
to the lofty altitude of more that ten miles above the 
general surface of the planet. By noting, likewise, the 
variation in the appearance of the horns of the planet, 
when it assumed a crescent sha$>e, the same astrono- 
mer ascertained to his own satisfaction the fact of its 



What is the inclination of its orbit to the plane of the ecliptic ? What is its actual 
diameter in miles? What is the periodic time of Mercury? State why it is dif- 
ficult to ascertain, with certainty, the physical characteristics of this planet. Are 
there many reliable observations on Mercury? State what is said respecting Sir 
William Herschel's efforts. 



118 ASTRONOMY, 



rotation; the period of which he estimated at 24h. 
5m. 28sec. Since the time of Schroeter no astrono- 
mer has gained any further information on these 
points, which future observations may modify or 
confirm. v 

202. Phases. On examining Mercury with the 
telescope in different points of his orbit, we find that 
he presents phases Uke those of the moon in her revo- 
lution about the earth. 

203. Transit of Mercury. If the plane of the 
orbit of Mercury was coincident with that of the 
ecliptic, the planet at every inferior conjunction, would 
pass directly between us and the dish of the sun, and 
would appear as a black spot upon it. But since the 
plane of its orbit is inclined to that of the ecliptic 
about 7 degrees, this phenomenon does not occur at 
every inferior conjunction, for the planet may be on 
one side of the disk of the sun when it is in this posi- 
tion. 

204. In order that a transit may occur, the earth 
must be in the line of the nodes of Mercury, at or very 
near the time when the planet passes through one of 
them, in its revolution about the sun. For Mercury 
being at the node, is consequently in the plane of the 
ecliptic, and the line of the nodes will then pass 
through the sun, the earth, and Mercury, and the 
latter, as seen from the earth, will be projected as a 
dark spot upon the sun ; just as the moon is during 

What success had Schroeter? What is the period of Mercury's rotation, as deter- 
mined by him? Have later astronomers increased our knowledge of the physical 
characteristics of Mercury ? What phenomenon is observed in respect to this planet, 
when viewed with a telescope ? Why does not a transit of Mercury occur at every 
inferior conjunction ? What must be the respective positions of the planet and the 
earth, that a transit may occur ? Why ? 



ASTRONOMY. 119 



a solar eclipse. If ike planet, the earth, and su?i, are 
not exactly in the line of the nodes, still a transit may 
occur within certain limits, on account of the magni- 
tude of the sun ; the planet crossing the disk of the 
sun, not through its centre, but on one side of it. 

205. The earth arrives at the line of the nodes 
twice a year, about the 10th of November and the 7th 
of May, and the transits of Mercury will for a long 
time happen in these months. The last transit oc- 
curred on the 8th of November, 1848, and the second 
after the next will happen on the 6th of May, 1878. 

206. Mass and Density. The investigations of as- 
tronomers in respect to these particulars have led to 
the conclusion, that the mass of the sun exceeds that 
of Mercury 4,865,750 times, and that the density of 
the planet is -g-th greater than that of the earth. 

Yenus. ? 

207. We now come to Venus, the second planet in 
order from the *sun, and the most beautiful star that 
adorns the heavens. Her mean distance from the 
sun is 68,770,000 miles, and she revolves about this 
luminary in 225^ days, or more accurately, 224 days 
16h. 49m. 8sec. 

208. The length of her diameter, according to the 
best observations, is about 7900 miles, which is very 
nearly the same as that of the earth. Unlike that of 
Mercury, the orbit of Venus is almost a circle, and 

If these three bodies are not exactly in the line of Mercury's node, can this pheno- 
menon occur? Why? In what months do the transits happen ? When did the last 
transit occur? When will the second after the next take place? State what is 
said respecting the mass and density of Mercury. What is said respecting Venus ? 
What is her distance from the sun? What hex periodic time ? What is the length 
of her diameter ? What is said of her orbit 1 



20 ASTRONOMY. 



the inclination of its plane to that of the ecliptic is 
about 3° 23'. 

209. Rotation. The intense splendor of Yenus in- 
vests every part of her disk with such a brilliant 
light, that any variation in the surface of the orb, for 
the most part, escapes detection, since the valleys, as 
well as the mountains, if such inequalities exist, are 
bathed in floods of light; and astronomers therefore 
speak doubtingly of cloudy spots upon the surface of 
the planet. 

210. It is usually by directing their observations to 
well-defined spots, that astronomers determine the 
period of the rotation of a planet upon its axis ; the 
absence of such marks upon Yenus, for a long time, 
rendered the time of her rotation a matter of uncer- 
tainty. 

At length Schroeter, the celebrated German astron- 
omer, ascertained that this orb revolved on its axis 
in 23h. 21m. and 8sec. This result has been almost 
universally received, though it is not regarded by 
astronomers as exact beyond the possibility of an 
error. 

211. Phases. In her revolution about the sun, 
Yenus, like Mercury, presents to our view similar 
phases to those of the moon. But since this planet 
is nearly twice as far from the sun as Mercury, and 
its real diameter is almost three times greater, these 

How much, is its plane inclined to that of the ecliptic? Why do we know 
scarcely anything respecting the surface of this planet? What is said in regard 
to the existence of spots ? How do astronomers ascertain the fact and time of a 
planet's rotation? State by whom the rotation of Venus was discovered, and the 
period of the same determined. Is this period of Venus' rotation considered by 
astronomers as absolutely exact? Describe the phases in full. 



ASTRONOMY. 121 



phenomena are niore conspicuous, and can be ob- 
served for a longer consecutive period. 

212. In a certain part of her orbit, we behold this 
beautiful planet rising a little before the sun, when 
it is termed the morning star. It has then just passed 
its inferior conjunction, and its dark side is turned 
towards the earth, like that of the moon when she is 
new; and it is now crescent-shaped. At its greatest 
angular distance from the sun, which is about 47^°, 
it appears like a half moon, and shines with great 
splendor. "When Venus arrives at her superior con- 
junction, she is seen like the full moon, her bright 
disk being nearly circular. 

Passing this point of her orbit, she rises after the 
sun, and of course sets after it, and is now the eve- 
ning star. 



FIG. 36. 




TELESCOPIC APPEARANCE OF VENUS WHEN NEAR HER INFERIOR 
CONJUNCTION. 

Fig. 36 is a representation of Yenus as she appears 

when viewed through a telescope near her inferior 

conjunction. 

6 



122 ASTRONOMY 



213. Splendor of Venus. Yenus shines with the 
greatest brilliancy when her angular distance from 
the sun is a little less than 40°. About once in eight 
years, under a favorable concurrence of circumstances, 
her splendor is usually great. The brightness of the 
planet is then so intense that under a serene sky it 
can be seen even at noon day. 

214. Transit of Yenus. This appellation is given, 
as in the case of Mercury, to the passage of Yenus 
across the sun's disk. A high importance is attached 
to this phenomenon by astronomers, since by means 
of it they are enabled to obtain with great accuracy 
the parallax of the sun, without which the distance 
of the earth from the sun could not be determined. 

215. The transits of Yenus, for a long time, will 
occur early in the months of June and December^ 
since the planet passes her nodes in the beginning of 
these months, and the motion of the nodes along the 
ecliptic is extremely small. They are, however, phe- 
nomena of rare occurrence, happening at intervals of 
about eight and one hundred and thirteen years. The 
next transit takes place on the 6th of December, 1882. 
None happens during the 20th century, the next oc- 
curring on the morning of the 7th of June, 2004, A.D. 

216. Mass — Density. From the latest and most 
accurate investigations, it appears that the sun con- 
tains 401,839 times more matter than Yenus. She 
has therefore a little less matter than the earth, since 
the mass of the sun is only 354,000 times greater than 

State what is said of the splendor of Venus. What is said in regard to the transits 
of Venus ? In what months of the year will the transits of Venus occur for a long 
while? Why? Are these phenomena frequent? When will the next transit 
take place ? What is the mass of Venus ? 



ASTRONOMY. 123 



that of the earth. The density of Venus nearly equals 

the density of the earth, the former being to the latter 

as 92 to 100. 

The Earth. © 

217. The next planet is the Earth. This, with its 
attendant moon, we have already discussed, and there- 
fore pass on to the superior planets. 

Superior Planets. 

218. These celestial bodies are more distant from 
the sun than the earth is, and their orbits conse- 
quently encircle that of the earth. They are in 
superior conjunction when the sun is directly he- 
tween them and earth, and in opposition when the 
earth is directly between them and the sun. As they 
can never come between the earth and the sun, it is 
of course impossible that they should have any inferior 
conjunction; on this account they are not subject to 
phases like those of Mercury and Venus. Moreover, 
they are seen at all angular distances from the sun, 
from 0° to 180°. In these three respects, as viewed 
from the earth, they differ from the inferior planets. 
The next planet in order is Mars. 

Mars. $ 

219. This planet is situated at the average dis- 
tance of about 145,205,000 miles from the sun, and 
the inclination of its orbit to that of the ecliptic is 

What her density"? What is the next planet in order? What is said of it? 
What is said respecting the superior planets ? Have they any inferior conjunction ? 
State the three particulars in which they differ from the inferior planets, as viewed 
from the earth. What is the name of the superior planet next in order ? What is 
the solar distance of Mars? What is his periodic time ? What is the inclination 
of its plane to that of the ecliptic ? 



124 ASTRONOMY. 



about 1° 53'. The period of time occupied by Mars 
in making one revolution about the sun, is, according 
to the best computation, 686 days 23h. 30m. 41sec. ; 
the time of its rotation on its axis 24h. 37m. and 
20sec. ; and the length of its diameter 4500 miles. 
Its density is nearly the same as that of the earth. 

220. Splekdor. Mars, when nearest to us, shines 
with great splendor, and rising about sunset, moves 
along the sky a conspicuous object throughout the 
night; but when most remote from the earth, he appears 
like a star of ordinary size. The cause of these great 
changes is readily perceived, when we consider, that 
inasmuch as the orbit of Mars includes that of the 
earth, his distance from the earth at superior conjunc- 
tion, equals his own distance from the sun, increased 
by that of the earth's solar distance, and at opposition 
it is only equal to the difference of these distances. 
Stating the same in figures, the distance of Mars from 
the earth at superior conjunction, amounts in round 
numbers to 145,000,000 miles added to 95,000,000 
miles, or 240,000,000 miles ; while at opposition it is 
equal to 145,000,000 miles diminished by 95,000,000 
miles, or 50,000,000 miles. A variation in distance 
so extensive as this, must, of course, give rise to cor- 
responding changes in the apparent size and brilliancy 
of the planet. 

221. Physical Aspect. "When viewed through a 
telescope of adequate power, the outlines of continents 
and seas are revealed on the surface of Mars, while 



What is the time of its rotation on its axis? What the length of its diameter? 
What is said of its density ? What is said respecting the changes in the splendorof 
Mars? Explain the cause of these variations. Describe the physical aspects 
of Mars. 



ASTRONOMY. 125 



near the poles white spots are discerned, which, from 
their increase and decrease, with the change of its 
seasons, have been regarded by Sir fm. Herschel, as 
masses of ice and snow that accumulate during the 
winter of Mars, and diminish in the summer. The 
continents appear of a dull red hue, while the seas 
possess a greenish tinge. The ruddy hue of the 
planet, by which it is easily distinguished from other 
heavenly bodies, is attributed by Sir John Herschel 
to the prevailing color of the land. 

FIG. 37. 




MARS, AS SEEN BY SIR JOHN HERSCHEL. 

222. Fig. 37 represents Mars as viewed by the ac- 
complished astronomer, Sir John Herschel, in his 20 
feet telescope, on the 16th of August, 1830. It shows 
the planet in its gibbous state, with the outlines of its 
continents and seas ; while one of the white spots 
which are situated near its poles, is distinctly discern- 
able on its surface. 

What does Fig. 37 represent ? 



126 ASTRONOMY. 



The Asteroids. 

223. The astronomer Kepler, 250 years ago, no- 
ticed a tendency to a regular progression in the 
distances of the planets from the sun, as far as 
Mars. Twice the distance of Mercury from the sun, 
is nearly the distance 'of Venus ; three times that of 
Mercury is about the distance of the earth ; and four 
times the distance of Mercury gives almost exactly 
the distance of Mars. But in order to represent the dis- 
tance of Jupiter, between which orb and Mars no planet 
in the time of Kepler was known to exist, the distance 
of Mercury must be multiplied, not by 5, but by 13. 

224. The law appeared here to be broken, and an 
immense interval of 350,000,000 miles, extending 
between Mars and Jupiter, to be unoccupied by a 
single planetary body. Kepler imagined that in order 
to preserve the harmony of distance, another planet 
existed in this vast space, which had hitherto eluded 
the searching gaze of astronomers. 

225. For two centuries nothing was done either to 
verify or overthrow this hypothesis of Kepler ; but 
when, in 1781, Uranus was discovered by Sir ¥m. 
Herschel, an impulse was given to astronomical in- 
vestigations, and an association of astronomers com- 
menced a systematic search for this supposed planet. 
Ere long, instead of one, four small planets were dis- 
covered, to which were assigned the names of Ceres, 
Pallas, Juno, and Yesta. 

What did Kepler remark in regard to the solar distances of the planets ? Where 
was this law broken ? What did this fact lead him to think ? Was anything done 
by the astronomers who immediately succeeded Kepler, to confirm or overthrow his 
hypothesis ? When was a new impulse given to astronomical research, and why? 
What was then done by astronomers? 



ASTRONOMY. 127 



226. Nearly 50 years more elapsed, when the search 
was renewed in the same region of space, and the dis- 
covery of 38 additional asteroids has rewarded the 
labors of the astronomer. 

227. A list of the asteroids, in the order of their 
discovery, is given on the following page, together 
with their distances, periodic times, and other partic-* 
ulars respecting them. 

228. From this list it appea^ that 42 asteroids have 
already been discovered between Mars and Jupiter. 
In the opinion of Leverrier, a distinguished French 
astronomer, all have not yet been found, and he thinks 
it probable that before the year 1860, as many as a 
hundred will have been discovered within this space. 

229. Dr. Olbers believed that a large planet once 
existed between Mars and Jupiter, that it was shat- 
tered into fragments by some tremendous convulsion, 
and that from these fragments the asteroids have been 

formed. 

Jupiter. U 

230. Next in order from the sun is Jupiter, the most 
magnificent planet that illumines the sky. Its peri- 
odic time is 4332 days, or somewhat more than twelve 
of our years. The average distance of Jupiter is 
495,817,000 miles, and his diameter, according to 
Prof. Struve, is 88,780 miles. 

231. The hulk of the planet is more than twelve hun- 
dred times greater than that of the earth. It revolves on 

What success has attended this search for planets? How many asteroids have 
been discovered ? What are Leverrier's views respecting their number? What is 
Dr. Olbers' theory ? What is the next planet in order from the sun ? What is his 
-periodic time 7 His solar distance ? What the extent of his diameter ? What is 
said respecting the bulk of this planet ? 



128 



ASTRONOMY 



TABLE OF ASTEROIDS. 



CERES ? 

PALLAS $ 

JUNO a 

VESTA t 

ASTREA m 

HEBE 1 

IRIS <S 

FLORA M 

METIS 4* 

HYGEIA (10) 

PARTHENOPE....(il) 
VICTORIA, or CLIO ^ 

EGERIA (13) 

IRENE *&, 



.(15) 

• (15) 
.(17) 
.(18) 

• (19) 

,(20), 
.(21) 

•(22) 

.(23). 
.(24). 

(25). 

[€S»> 

(27). 



EUNOMIA.... 

PSYCHE 

THETIS 

MELPOMENE 
FORTUNA . . . 
MASSALIA . . 
LUTETIA .... 
CALLIOPE... 

THALIA 

THEMIS 

PHOCEA 

PROSERPINE. 
EUTERPE.... 

BELLONA (28). 

AMPHITRITE (29) 

URANIA (30) 

EUPHROSYNE....(3I) 

POMONA (32) 

POLYMNIA (33) 

CIRCE (34; 

LEUCOTHEA (35) 

FIDES (30) 

ATA'LANTA.......(37) 

LED A (38) 

LiETITIA (39) 

HARMONIA (40) 

DAPHNE (41) 

ISIS (42) 



DISTANCE 
FROM THE 

SUN. 



Milea. 

262,747,675 
263,105,635 
253,518,045 
224,253,105 
244,853,190 
230,327,785 



Bays 
1680 
1683 
1592 
1325 
1511 
1379 



226,718,839 1 34? 



209.131,670 
226,700,875 
299,191,480 
232.569,215 
221,794,600 
244,804,550 
245,536,620 
251,123,950 
278,630.345 
235.971,260 
217,890.100 
232,187,030 
228,139,650 
231,240,070 
276,612,450 
251.286,780 
298,727,120 
228,063,935 
245,841,190 
223,046,035 
264,168,875 
241,898,215 
224,041,065 
303,267,265 
245,580,320 
225,985,050 
253,916,000 



1193 
1346 
2041 
1399 
1303 
1512 
1518 
1570 
1835 
1430 
1269 
1396 
1359 
1387 
1815 
1571 
2037 
1359 
1522 
1314 
1694 
1484 
1323 
2083 
1518 
1340 
1596 



247,399,000 



262,909,500 
215,441,000 



217,531,000 



1535 



1682 
1247 



1265 



BY "WHOM DISCOVERED. 



Piazzi, of Palermo, 

Olbers, of Bremen, 

Harding, of Lilienthal,. 

Olbers, of Bremen, 

Hencke, of Dnessen, 

Hencke, of Driessen, 

J. R. Hind, of London,., 
J. R. Hind, of London,.. 

Graham., of Markree, 

Gasparis, of Naples, 

Gasparis, of Naples, 

J. R. Hind, of London,.. 

Gasparis, of Naples, 

J. R. Hind, of London,. . 

Gasparis, of Naples, 

Gasparis, of Naples, 

Luther, of Bilk, 

J. R. Hind, of London,.. . 
J. R. Hind, of London^. . 

Gasparis, of Naples. 

Goldschmidt, 

J. R. Hind, of London,. . . 
J. R. Hind, of London, . . . 

Gasparis, of Naples, 

Chacornac, of Marseilles. 

Luther, of Bilk 

J. R. Hind, of London,.. 

Luther, of Bilk, 

Marth, of London, 

J. R. Hind, of London, . . 
Ferguson, of Wash., D.C., 
Goldschmidt, of Paris,. . . 

Chacornac, of Paris, 

Chacornac, of Paris, 

Luther, of Bilk, 

Luther, of Bilk, 

Goldschmidt, of Paris, . . . 

Chacornac, of Paris, 

Chacornac, of Paris, 

Goldschmidt, of Paris, . . . 
Goldschmidt, of Paris,... 
Pop^on, of Oxford, 



Jan. 1801 
Mar. 1802 
Sept. 1804 
Mar. 1807 
Dec. 1845 
July, 1847 
Aug. 1847 
Oct. 1847 
Apr. 1848 
Apr. 1849 
May, 3.850 
Sept. 1850 
Nov. 1850 
May, 1851 
July. 1851 
Mar. 1852 
Apr. 1852 
June, 1852 
Aug. 1852 
Sept. 1852 
Nov. 1852 
Nov; 1852 
Dec. 1852 
Apr. 1853 
Apr. 1853 
May, 1853 
Nov. 1853 
Mar. 1854 
Mar. 1854 
July, 1854 
Sept. 1854 
Oct. 1854 
Oct. 1854 
Apr. 1855 
Apr. 1855 
Oct. 1855 
Oct. 1855 
Jan. 1856 
Feb. 1S56 
Mar. 1856 
May, 1856 
May, 1856 



Miles 

163 

670 

295 



ASTRONOMY. 129 



its axis in a little less than 10 hours (9h. 55m. 30sec.), 
and it contains about one thousandth part of the 
amount of matter that is contained in the sun. 

232. Physical Aspect of Jupiter — Belts. When 
this beautiful planet is seen through a telescope, no 
configurations are beheld on its surface, marking the 
positions of continents and seas, as is the case of Mars, 
but dcvrk bands, termed belts, are seen, stretching from 
side to side in the same direction. They are by no 
means uniform in their appearance ; and, although 
for months they sometimes remained unchanged, they 
are yet liable to sudden and extensive alterations in 
their breadth and situation, though not in respect to 
their general direction. In a few rare instances, they 
have been seen broken up and distributed over the 
entire dish of the planet. Branches are frequently 
observed diverging from the main belts, and dark 
spots have likewise been noticed, of which astrono- 
mers have availed themselves to ascertain the period 
occupied by the planet in revolving on its axis. 

233. The views generally entertained by astrono- 
mers in respect to the cause of the belts, are the fol- 
lowing : It is supposed that Jupiter is surrounded by 
a luminous atmospheric envelope, which conceals, for 
the most part, the planet itself; and that this bright 
canopy is parted by narrow openings parallel to the 
equator of Jupiter. That an observer on the earth 
looking through these openings sees the dark surface 
of the planet, and that the glimpses thus caught of the 
solid body constitute the narrow dusky bands or belts. 

What of its rotation ? How much matter does it contain ? Describe the belts 
and their changes. What is the prevailing opinion of astronomers as to the cause 
of the belts ? 

6* 



130 ASTRONOMY. 



234. These rents in the atmosphere of Jupiter are 
supposed to be caused by currents, like our trade 
winds, but vastly more powerful, owing to the im- 
mense velocity with which the planet rotates ; and the 
variations in the action of these winds upon the 
atmosphere of the planet would account for the 
changes that are noticed in the aspect of the belts. 
The appearance which Jupiter displays when seen 
through a telescope, is shown in Fig. 38. 

FIG. 38. 



JUPITER AND HIS BELTS. 

235. Satellites of Jupiter — Their Discovert. A 
splendid train of four moons, or satellites, are seen by 
the aid of the telescope circling around this planet. 
They were discovered by Galileo of Padua, on the 
8th of January, 1610, and were the first fruits of his 
invention of the telescope. From that time to the 
present they have ever engaged the attention of 

How are the changes in the appearance of the belts supposed to arise ? How many 
moons has Jupiter? By whom, when, and how were they discovered ? Why are they 
regarded with interest by astronomers ? 



ASTRONOMY. 131 



astronomers, and their eclipses have been eminently 
serviceable in certain scientific investigations. 

236. Their Magstptudes — Diameters — Distances — 
and Periods of Revolution. No names have been 
given to these moons, but they are denominated the 
first, second, third, and fourth satellites, according 
to their distances from Jupiter, the first being the 
nearest. Their respective diameters, distances, and 
periods of revolution around Jupiter, are given in 
the table below : 

Diameter. Dist. from Juptter. Periods of Revolution. 
First Satellite, 2,440 miles, 278,500 miles, Id. 18h. 27m. 34sec. 
Second «« 2,190 " 443 000 " 3d. 13h. 14m. 36sec. 

Third " 3,580 " 707,000 " 7d. 3h. 42m. 33sec. 

Fourth, " 3,060 " 1,243,500 " 16d. 16h. 31m. 50sec. 

The first two satellites are larger than our moon, 
and the last two greater than the planet Mercury — 
the diameter of the third exceeding that of Mercury 
by 630 miles. 

237. Kepler's Laws — Eotation. The satellites in 
their respective distances from the planet Jupiter, 
and in their periodic times, obey the third law of 
Kepler — the squares of their periodic times being as 
the cubes of their distances from their common pri- 
mary. An extended series of observations upon the 
periodical changes in their light, led Sir William 
Herschel to infer that each of the satellites revolves 
on its axis in exactly the same time as it completes 
one synodical revolution about Jupiter, thus following 
exactly the same law as our moon does in respect to 
the earth. 

State their magnitudes, diameters, distances, and periodic times. How do they 
compare in their actual dimensions with our moon and Mercury ? Does Kepler's 
third law apply to the satellites 1 State what is said in regard to their rotation. 
In what direction do the satellites revolve about Jupiter? 



'^ 



132 ASTRONOMY. 



238. Transits and Eclipses of the Satellites. The 
satellites revolve about Jupiter from west to east, and 
in planes nearly coincident with each other. They 
are, therefore, seen ranging together in almost a 
straight line, and seem to move backwards and for- 
wards in the heavens, , now passing in front of the 
planet, and now behind it. 

239. When they pass before the planet, their tran- 
sits occur, and they cast shadows upon their primary, 
which appear as da?*Jc spots crossing its bright disk. 

240. In passing behind the body of the planet, or 
into its shadow at a distance from it, the satellites 
disappear and their eclipses occur. The three satel- 
lites which are nearest to Jupiter are totally eclipsed, 
every revolution around their primary, but the fourth, 
from the greater inclination of its orbit, sometimes 
escapes being eclipsed, yet so seldom that its eclipses 
may be regarded as happening, for the most part, at 
every revolution, like those of the others. 

Saturn, h 

241. The next planet is Saturn, a vast globe, in- 
ferior in magnitude only to Jupiter, but surpassing 
it in the wondrous structure of its system, for Saturn 
is attended by a train of no less than eight satel- 
lites, and is girdled by several rings of stupendous size. 
Its average distance from the sun is about 900,000,000 
miles, and it revolves around it in 291 years. 

242. The equatorial diameter of Saturn is 77,000 
miles, and it contains about one thirty-five hundredth 

Why are they seen in a straight line with each other? How do they appear to 
move in the heavens? When do their transits occur? Describe them. Under 
what circumstances do their eclipses happen ? State what is said of their frequency. 
What planet is next discussed ? What is said of its grandeur ? What is its solar 
distance and periodic time ? What is the length of its diameter? 



ASTRONOMY. 133 



part of the amount of matter existing in the sun. 
From the observations of Sir William Herschel, who 
watched this planet through a hundred rotations, it 
appears that it revolves on its axis in 10 hours, 16 
minutes, and 4 seconds. 

243. Physical Aspect. Saturn appears of a pale 
yellowish hue, and when viewed through a good tele- 
scope, belts are frequently seen upon its surface, but 
far more faint and obscure than those which are re- 
vealed upon the disk of Jupiter. Spots are rarely 
noticed on this planet. 

244. 'Ring of Satuun — Its Discovery. When Gali- 
leo, in the year 1610, directed his telescope to Saturn, 
the figure of the planet appeared so singular, that he 
thought it consisted of a large globe, with a smaller one 
on each side. About 50 years afterwards, Huyghens, a 
distinguished Dutch philosopher, observed Saturn with 
telescopes of greater magnifying power than those 
which had been employed by Galileo, and soon made the 
discovery that the planet was surrounded by a vast lu- 
minous ring, unconnected with the body of the planet. 

245. When the telescope had been still farther im- 
proved, and instruments of higher magnifying powers 
and finer construction were at command, two English 
gentlemen of the name of Ball, in October of the year 
1665, first noticed that the ring was double; a pheno- 
menon which was observed by Cassini, at Paris, 1675, 
and to whom the honor of this second discovery is 
usually attributed. 

246. Form — Constitution. The whole ring may be 
described as circular, broad, and fiat, like a coin with 

How much matter does it contain 1 What is the period of its rotation 1 Describe 
the physical aspects of this planet. Give an account of the discovery of Saturn ? s 
ring. What is said respecting its form and mnstHvtinv? 



134 



ASTRONOMY 



a round central opening. Like the planet, it shines 
by the reflected rays of the sun, and has usually been 
supposed by astronomers to consist of solid matter, 
since it casts a shade upon the surface of the planet, 
when it is situated between the latter and the sun. 
Professors Pierce and Bond, of Harvard University, 
have, however, arrived at the conclusion that the ring 
of Saturn is not solid, hut fluid. Professor Pierce re- 
marks, "that the ring of Saturn consists of a number 
of streams of some fluid about one fourth heavier than 
water, flowing around the planet." 

247. Rotation — Position. From the observations 
made upon certain spots on its surface, Sir William 
Herschel inferred that the ring rotated in its own 
plane in the space of lOh. 32m. 15sec. The plane 
of the ring maintains invariably the same position in 
space, as is seen in Fig. 39. 

FIG. 39. 




SATURN AND HIS RING. 



248. Divisions of the Ring. We have just alluded 
to the discovery made by the Messrs. Ball, and also 
by Cassini, that the ring of Saturn is double. For 
nearly a century, astronomers have been led to think, 
from the appearance of dark lines upon the ring, that 
other subdivisions exist, and these surmises have 
proved correct. 

What of its rotation and position ? Relate in full the discoveries that have been 
made in respect to the divisions of the ring. 



ASTRONOMY. 135 



249. In 1837, Prof. Encke, of Berlin, saw, through 
the famous telescope of Fraunhofer, the outer ring of 
Saturn divided by a black line, and so clearly defined 
that he was enabled to take the measurements of its 
breadth. This separating line was observed some 
years afterwards by Messrs. Lassel, Dawes, and Hind, 
and also by Prof. Ohallis, of Cambridge University, 
England, and with such marked distinctness as to 
leave no doubt of the actual division of the outer ring. 

250. But this discovery was soon followed by 
another still more surprising, which was no less than 
the detection of a dusky obscure ring, nearer to the 
planet than what is usually termed the bright inner 
ring. On the 11th of November, 1850, Mr. G. P. 
Bond, of Harvard University, saw such evidences of 
subdivision in the inner ring as led him to infer that 
a third ring existed nearer the planet, and less bright 
than the other two. On the 29th of the same month, 
the Rev. W. R. Dawes, of Wateringbury, England, 
made the same discovery, and noticed, likewise, the 
additional fact, that the dusky ring is itself double, 
being divided by an extremely line line. 

251. What, therefore, was at first regarded as a 
single ring, is now found to consist of Jive, viz., two 
obscure rings nearest the planet, and three bright ones 
beyond them. The two exterior luminous rings con- 
stitute what has hitherto been termed the outer ring 
of Saturn, and the third the inner ring. Fig. 40 
represents Saturn and his rings, as they appeared to 
Mr. Dawes, of Wateringbury, when viewed through 
a telescope of the finest construction. The division 
of the dark inner ring is, however, not delineated. 

How many rinjr? have Ven found 7 



136 ASTRONOMY 



FIG. 40. 




SATURN, AS VIEWED BY THE REV. W. R. DAWES, ON NOVEMBER 29TH, 1850. 

252. Dimensions of the Rings. The dimensions of 
the outer and inner 1 rings of Saturn have been deter- 
minedj by the most accurate and careful measure- 
ments, to be as follows : 

From the surface of the planet to the inner edge of ) , ft ~ 2fi ., 

the first bright ring, j 

Breadth of the inner ring, 16,755 " 

Breadth of the interval between the bright inner | , „r 2 it 

and outer ring, ) 

Breadth of the outer ring, 10,316 " 

Outer diameter of the outer ring, 172,130 <4 

253. The thickness of the rings has been estimated 
by Sir John Herschel, at not more than 100 miles, 
while Mr. G. P. Bond, of Cambridge, places the thick- 
ness as low as 40 miles. 

1. Outer and inner ring. By the outer ring is here meant, as stated 
in the preceding article, the two exterior bright rings. The inner ring 
is the third bright ring, next to the dark one. 

Give the dimensions of the outer and inner rings of Saturn. What is the thick- 
ness of the rings, according to Sir John Herschel ? What, according to Mr. G. P. 
Bond? 



ASTRONOMY. 137 

254. Satellites of Saturn. Saturn is attended by 
eight moons, seven of which revolve about the planet 
in orbits whose planes are nearly coincident with that 
of the ring. They have received the names of Mimas, 
JEnceladus, Tethys, Dione, Rhea, Titan, Hyperion, 
and Japetus. ' 

255. On account of their great distance from the 
earth, these bodies, although possessed of considerable 
size, are only visible by the aid of powerful tele- 
scopes. 

256. Hyperion. This satellite was discovered as late 
as September, 1848, and almost at the same time by 
two observers. Mr. G. P. Bond, of Harvard Univer- 
sity, detected it on the 16th of September, and Mr. 
Lassel, of Liverpool, on the 18th of the same month. 

Uranus, or Herschel. *$ 

257. Until the year 1781, all the known planets, ex- 
cluding our earth, were Mercury, Venus, Mars, Jupiter, 
and Saturn, Each of these, more or less conspicuous 
to the unaided eye, had been recognized as planets for 
ages, but about this time, Sir William Herschel, having 
constructed telescopes of great power, commenced a 
systematic examination of the heavens, which led to 
the most surprising discoveries. 

258. On the 13th of March, 1781, between ten and 
eleven o'clock, this eminent astronomer detected an 
object which he at first suspected to be a comet, 
but subsequent observations established its planetary 



How many moons has Saturn? What is the position of the planes of the orbits 
of seven 7 Gjve the names of the satellites. What is said ?.s to their visibility ? 
Give an account of Hyperion. When was Uranus discovered, and by whom ? 



138 ASTRONOMY. 



nature. The new planet was called by Herschel, 
Georgium Sidus, as a compliment to his patron, 
George III., and by others, Herschel, in honor of the 
discoverer ; but the name proposed by Bode, of Urcir 
nus, is now universally adopted. 

259. Aspect — Diameter — Mass. Uranus appears 
of a pale color, uniformly bright, and undiversified 
with spots, belts, or configurations of surf ace, such as 
are seen on Jupiter and Mars. Its diameter is about 
35,000 miles. According to the recent calculations 
of Mr. Adams, the sun contains 21,000 times as much 
matter as Uranus. The density of Uranus is exactly 
the same as that of Jupiter, or about one fourth of 
that of the earth. 

260. Rotation. The absence of spots and outlines 
upon the unvarying bright surface of Uranus, de- 
prives astronomers of the means of determining the 
period of its rotation. In fact, whether it revolves at 
all upon its axis, is a point not yet fully determined, 
but as it belongs to a system of planets,, all the rest 
of which revolve on their axes, it is reasonable to 
infer from analogy that Uranus also does. 

261. Distance — Periodic Time. The average dis- 
tance of Uranus from the sun is 1,828,071,000 miles, 
and it revolves about the sun in a little more than 84 
of our years. 

262. Satellites of Uranus. Uranus was found by 
Sir William Herschel to be attended by six satellites, 
but, notwithstanding the zealous efforts of astrono- 



What are the various names of this planet ? State what is said in regard to its 
aspect, diameter, and mass. What of its rotation, distance, and periodic time. 
How many satellites has Uranus ? 



ASTRONOMY. 139 



mers, little certain knowledge has yet 'been gained in 
respect to them. 

263. The satellites of Uranus differ in two particu- 
lars from all the other planetary bodies that compose 
the solar system. For all the planets and their satel- 
lites, excepting those of Uranus, revolve in their 
orbits from west to east, and the planes of their arhits 
do not deviate far from the plane of the ecliptic ; but 
the attendants of Uranus move around the planet 
from east to west, and the planes of their orbits are 
nearly perpendicular to the plane of the ecliptic. 

Neptune. J 

264. History or its Discovery. When an as- 
tronomer knows perfectly all the elements of a planet, 
he can tell at what time it will be in a particular 
place in the heavens, with greater precision than 
the station-master of a railroad can tell when a cer- 
tain train will arrive at a given station. If the planet 
does not arrive at its appointed place at the computed 
time, it must be owing to some influence unknown to 
the astronomer, provided he has made no error in his 
calculations. Now Uranus, ever since its discovery, 
has not kept its appointments, for astronomers have 
been constantly finding it in a different place from 
that in which it ought to have been according to their 
calculations. It was always off the track, and they at 
length suspected that these deviations were caused by 
the attraction of a planet hitherto undiscovered. 

265. Mr. Adams, of St. John's College, Cambridge, 

What do we know respecting them? Have they any peculiarities? What are 
they ? What is the next planet in order ? Give the history of the discovery of 
Neptune. 



140 ASTRONOMY. 



in 1843, and Mr. Leverrier, of Paris, in 1845, un- 
known to each other, undertook the task of solving 
this intricate problem, calculating how large a planet 
would account for these deviations, what distance it 
must be from the sun, what orbit it must have, and 
various other particulars. In September, 1846, the 
French astronomer had. so fully completed his com- 
putations, that on the 23d of the month, he wrote to 
Dr. Galle, of Berlin, telling him where to look in the 
heavens for the unknown planet, and of what size it 
would appear. Dr. Galle, the same evening he re- 
ceived the letter, pointed his instrument to that 
region in the heavens where he had been directed 
to gaze, and there he immediately saw a star of the 
magnitude mentioned by Leverrier, and which proved 
to be the planet sought. 

266. Name — Diameter — Mass — Density. The 
planet of Leverrier has generally received from 
astronomers the name of Neptune. Its diameter, de- 
duced from measurements made with the best instru- 
ments of Europe, is 31,000 miles. Its mass is not yet ac- 
curately known, but from the computations of several 
very able astronomers, it is ascertained that the sun 
contains about 18,000 times more matter than Neptune. 

267. Orbit — Distance — Periodic Time. The most 
accurate determination of Neptune's orbit is that 
made by Mr. Sears C. Walker, of Philadelphia. Ac- 
cording to this astronomer, the average solar distance 
of Neptune is 2,862,457,000 miles, and its periodic 
time about 164^ years. 



What is said respecting the name of this planet ? What of its diameter, mass, 
solar distance, and periodic time ? 



ASTRONOMY. 141 



268. Has Neptune a Ring ? Mr. Lassel, of Liver- 
pool, and Prof. Challis, of Cambridge, England, have 
at various times supposed that they saw traces of a 
ring surrounding the planet. Prof. Bond, of Cam- 
bridge, has frequently noticed a luminous appendage, 
but not so defined as to enable him to announce the 
existence of a ring. 

269. The Satellite of Neptune. In about a month 
after the discovery of Neptune by Dr. Galle, Mr. 
Lassel, of Liverpool, detected a satellite at about the 
same distance from the planet as the moon is from 
the earth. 



CHAPTER VI. 

COMETS. 



270. Comets are. a class of bodies belonging to the 
solar system, entirely different in appearance from 
any we have yet considered. The orbits in which 
they revolve are so elliptical that during the greater 
part of their circuit they are invisible, being only de- 
tected when near the sun. 

271. Constitution. The comet, when entire, con- 
sists of three parts — the head or nucleus, the coma 
or envelope, and the tail. The head is nearest to 
the sun, and appears as a bright spot, more dense 
than the other portions. Surrounding the head, but 

Has Neptune a ring 1 What is said of its satellite ? What does Chapter VI. 
treat of? What is said respecting these bodies 1 Of how many parts does a comet 
consist 7 Describe each of them in full. 



142 ASTRONOMY. 



yet perhaps separated from it, is the coma, which is a 
luminous, fog-like covering, that probably conceals 
from our view the real body of the comet. This 
envelope is conceived to give to comets a hairy 
appearance / hence their name. 1 

272. The tail is an expansion of the coma, the light 
matter of which, streaming backward on either side 
in a direction opposite to the sun, diffuses itself, for 
the most part, into two broad trains of light, extend- 
ing to an immense distance, and which constitute the 
tail. All comets do not possess tails ; even some of 
the most conspicuous present to view tails of only 
moderate dimensions, while others are as perfectly 
free from them as a planet. 

273. In Fig. 41, where the comet of 1819 is delin- 
eated, its three distinct parts are easily recognized. 

274. Number of Comets. This class of celestial 
bodies is without doubt very numerous, for, accord- 
ing to Sir John Herschel, the list of those on record, 
before the invention of the telescope, amounts to 
several hundred. M. Arago has considered that he 
might safely estimate the number of comets with- 
in the orbit of Uranus at 7,000,000, and there are 
probably within the orbit of Neptune more than 
28,000,000. 

275. Splendor and Size. Comets vary much in 
respect to their brilliancy and magnitude ; for, while 
multitudes are only visible through the telescope, 

1. Comety from the Greek word home, signifying hair. 

Do all comets possess tails or trains ? What is said respecting the number of 
comets on record before the telescope was invented ? What was Arago's estimate 
of the number? How many are there probably within the orbit of Neptune? 
What is said of the splendor and size of comets ? 



ASTRONOMY. 143 



FIG. 41. 




COMET OF 1819. 

many of which are destitute of tails and heads, ap- 
pearing only as cloudy stars, others almost dazzle the 
gaze with their brightness, and extend their bright 
tails half across the heavens. Some comets have 
been seen of such surpassing splendor that they were 
visible in clear daylight. 

276. Comet of 1680. The famous comet of 1680 
was conspicuous for the great length of its tail ; for 
soon after its nearest approach to the sun, this won- 
drous appendage shot out from the body of the comet 
to the distance of 60,000,000 miles, and in the in- 
credible short space of two days. When it had 
attained its greatest length, it extended no less than 
123,000,000 miles from the head, covering a space in 
the heavens greater than the distance from the 
horizon to the zenith. So swiftly did it move that it 

Describe the comet of 1680. 



144 ASTRONOMY. 



is said to have gone half around the sun in ten and a 
half hours, moving with the speed of 880,000 miles 
an hour. 

This comet came within' the distance of only 
147,000 miles from the surface of the sun, and was 
exposed to a heat 27,500 times greater than that 
received by the earth in the same time — a heat 2000 
times greater than that of red-hot iron. 

277. Comet of 1843. This comet w^as seen on the 
28th of February, 1843, close to the sun, its bright- 
ness being so great that the splendor of the solar 
beams could not overpower its brilliancy. The diam- 
eter of its envelope was 36,000 miles, and the greatest 
length of the train 108,000,000 miles. 

At the Cape of Good Hope it appeared on the 3d 
of March to be double, two trains diverging from the 
head in a straight line, forming a small angle with 
each other. Near the equator this magnificent ap- 
pendage shone with such a glow that at times it 
threw a bright light upon the sea. It swept more 
than half around the sun in two and a half hours, 
moving with a velocity of 1,300,000 miles an hour. 
It came within about 60,000 miles of the sun's sur- 
face, and, according to Sir John Herschel, the heat 
it received from the sun was 47,000 times greater 
than that which falls upon the earth in the same 
time, when the sun is shining perpendicularly upon 
it. So intense is such a heat, that it is 24J times 
greater than that which is sufficient to melt agate or 
rock crystal. 

278. Orbits. The orbits of comets, for the most 

Describe the comet of 1843. State what is said respecting the orbits of comets. 



ASTRONOMY. 145 



part, are ellipses, with the sun in their common focus ; 
but, unlike those of the planets, which deviate but 
little from a circle in form, the elliptical orbits of 
comets are exceedingly elongated. 

In consequence of this extended form of the orbit, 
the comet is only beheld for a short time while it is 
near the sun; after which it occupies years, and even 
centuries, in accomplishing the remainder of its cir- 
cuit, sweeping far beyond the limits of the planetary 
system, where no telescope can begin to descry it. 

279. The Comets of Halley, Encke, Biela, and 
Fate. Of all the comets that have been observed, the 
orbits of about 190 have been determined, and out of 
all these, the return of ■ only four have been verified 
by observation, namely: Halley's, Encke's, Biela's, 
and Faye's. Halley's comet returns at intervals of 
about 75 or 76 years. Its last appearance was in 
1835, when its predicted corresponded with its actual 
return within one day. It recedes from the sun 
600,000,000 miles beyond Neptune. The period of 
Encke's comet is 3J years, that of Biela's, 6| years, 
and that of Faye's, 7| years. 

280. Nature of Comets. These extraordinary 
bodies consist of matter, but existing in an attenuated 
and diffused state, of which we have no adequate con- 
ception. A light cloud, in comparison with the 
matter composing the tail of a comet, is to be regarded 
as a dense and heavy body. 

281. The amount of matter in comets, even of the 
largest size, is so small that their passage around the 

Of how many comets have the orbits been determined? Of how many has the 
return been verified by observation ? Describe Halley's comet, Encke's, Biela's, 
and Faye's. What is said of the vature of comets, and their amount of matter? 



146 ASTRONOMY 



sun has never, in the least perceptible degree, affected 
the stability of the solar system / in other words, they 
have never, as far as could be perceived, caused the 
planets to deviate a hair's breadth from their accus- 
tomed paths around the sun. 



CHAPTER VII 

TIDES. 



282. Him periodical rising and falling of the waters 
of the ocean, in alternate succession, are called tides. 
Standing on the sea shore, a person will perceive that 
for the space of nearly 6 hours, the waters of the sea 
continue to rise higher and higher, overflowing the 
shores, and running into the channels of the rivers. 
When they have attained their greatest elevation, it 
is then said to be high tide, full sea, or flood tide. 
Remaining at this elevation only for a few moments, 
they then begin to fall, and continue to sink for about 
6 hours more. When the waters have reached their 
greatest depression, it is then low, or ebb tide. After 
attaining this point, the sea, in a short time, again 
begins to swell, in the same manner as before, and 
thus, from year to year, and from cenhcry to century, 
the ebb and flow of the ocean follow each other at 
regular intervals of time. 

283. From the above explanation, it will be seen 
that there are daily two high tides and two low tides. 

What does Chapter VII. treat of? What are the tides! Describe them, explain- 
ing the meaning of high tide and low tide. How many high tides and low tides oc- 
cur daily? 



ASTRONOMY. 147 



The interval of time between two successive high or 
low tides, is about 12h. 25m. Accordingly, when 
there is a high tide at any place, as Xew York, for 
instance, there must also be a high tide on the oppo- 
site side of the globe ; and the same is true in respect 
to a low tide. 

284 A marked correspondence exists between the 
motion of the tides and the motion of the moon. If 
to-day, at 10 A.M., it is high tide in a certain harbor, 
it will be high tide to-morrow in the same harbor at 
lOh. 50m. 28sec. A.M. The interval, therefore, that 

FIG. 42. 



THE TIDES. 



elapses between any high tide and the next but one 
after it, is 24h. 50m. 28sec. JSTow, this is the exact 
amount of time that intervenes between two successive 
passages of the moon over the meridian of any place. 
In fact, as the earth revolves on her axis, the tide wave 
tends to keep under the moon, and thus sweeps around 

What is the interval of time between two successive high or low tides? When 
a high tide, for instance, occurs at any port, where is there then also another high 
tide?. What marked correspondence is here alluded to? Describe it particularly. 



148 ASTRONOMY. 



the globe, from any port to the same port again, in 
the precise period of time that elapses between two 
successive returns of the moon to the meridian of this 
port. This subject is illustrated by Fig. 42. 

285. Cause of the Tides. The unequal attraction 
exerted by the sun and moon upon different parts of the 
globe, produces the tides, and we will now proceed to 
explain this phenomenon, commencing with the moon. 

The waters of the earth directly under the moon, 
are more attracted by the moon than the solid part of 
the earth, because they are nearer (Art. 189), and a 
high tide is therefore produced about under the moon. 
The solid part of the earth is nearer to the moon than 
the waters on the side of the earth 'opposite to the 
moon ; the solid part is, therefore, more attracted than 
these waters, and is, as it were, drawn away from 
them by the moon, thus creating a high tide on the 
side of the earth opposite to the moon. Half-way 
between the two high tides, are the two low^ tides. 

286. Solar Influence. The sun, Kke the moon, 
produces tides by the unequal attraction it exerts 
upon the waters of the ocean, causing high tides at the 
points immediately beneath it, on opposite sides of the 
globe ; and low tides half-way from these points. The 
sun's influence is, however, only about one third of 
that of the moon, notwithstanding its vast superiority 
in size and mass. But any difficulty that may arise 
in understanding this fact, will vanish, when we re- 
flect that it is the unequal action of these bodies upon 



What is the cause of the tides ? Explain the action of the moon in producing 
tides. What is said respecting the sun's influence in producing tides? What is 
said of the amount of solar influence? 



ASTRONOMY. 149 



the waters of the earth that produces the tides, and 
not their whole attraction. Now, the waters of the 
globe just under the sun and moon, are about 8000 
miles (the earth's diameter) nearer the sun and moon 
than the waters on the opposite side ; but 8000 miles 
is g^n part of the moon's distance from the earth , 
while it is only T 2o~oo tn P art of the sun's distance from 
the earth. 

287. Spring and Neap Tides. We have just seen 
that the sun and moon cause tides in the ocean, inde- 
pendently of each other. These bodies, however, are 
perpetually changing their relative positions in the 
heavens, and on this account their separate actions are 
at alternate periods of time united and opposed to each 
other. The sun and moon act together twice a month, 
viz., at the opposition and conjunction, and the tides 
are then unusually high, since the lunar and solar tide 
waves are then heaped one upon the other. These are 

the SPRING TIDES. 

288. Twice every month, at the quadratures, the 
sun and moon oppose each other ; for at those points 
on the earth's surface where the surfs action then 
tends to elevate the waters, the moon's influence de- 
presses them, and where the moon raises the surface 
of the ocean, the influence of the sun is exerted to 
cause it to sink. These are the neap tides. 

289. The height of the lunar tide wave being about 
5 feet, and the solar 2, the average heights of the 
spring and neap tides will be in the ratio of 7 to 3. 
At the time of the neap tides, the loio tides are higher 
than ordinary, since at the places where they occur, 

Explain why it is small. When do the spring tides occur 1 When the neap tides ? 



150 



ASTRONOMY. 



the solar tide wave is at its greatest altitude, and its 
height must be added to the height of the low water, 
caused by the moon's action. But the high tides are 
then unusually law, since the lunar high tide wave is 
diminished by the solar low tide. 




SPRING TIDE NEW MOON. 



290. In Figs. 43 and 44, the subject of the spring 
tides is illustrated. In each of these figures, S repre- 
sents the sun, M the moon, and E the solid portion of 




SPRING TIDE FULL MOON. 



the earth. The dotted line inclosing the earth, is the 
solar tide wave, and upon this, in the line of the three 
bodies, is heaped the lunar tide wave, the boundary of 
which is the outer curved line. 



Describe these phenomona in full, and explain from Figures 43, 44, and 45. 



ASTRONOMY 



151 



291. In Fig. 45 is exhibited the phenomenon of the 
neap tides. The moon is in quadrature, 90° from the 
sun, and the two bodies evidently counteract each 
other's influence in producing their respective tides. 
The solar tide wave, as in the preceding figure, is rep 

FIG. 45. 





NEAP TIDE QUADRATURE. 



resented by the dotted oval line, and the lunar tide 
wave by the unbroken curved line. 

292. Actual Heights of the Tide. The theoretical 
height of the tide does not correspond to the real 
height. This difference is owing to local causes, such 
as the union of two tides, or the rushing of the tide 
wave into a narrow channel. In the latter case, the 
advance of the tide is often very rapid, and the water 
rises to a great elevation. Thus within the British 
Channel, the sea is so compressed that the tide rises 
50 feet at St. Malo's, on the coast of France. In the 
Bay of Fundy, the tide swells to the height of 60 or 

Why does not the theoretical height of the tide in any place correspond with the 
actual height ? State the cases cited. 



152 ASTRONOMY. 



70 feet. Here, according to Prof. Whewell, the tide 
wave of the South Atlantic meets the tide wave of the 
Northern Ocean, and their union raises the surface of 
the sea to the height just mentioned. On the vast 
Pacific, where the great tide wave moves without ob- 
struction, the rise of the water is only about two feet 
on the shores of some of the South Sea Islands. 

293. No Tides except on the Ocean, and on Seas 
connected with it. Inland seas and lakes have no 
perceptible tides. None have ever been observed in 
the Caspian sea, or in any of the great North Ameri- 
can lakes. This is owing to the fact, that the attract- 
ive forces exerted by the moon upon the waters of a 
lake, are so nearly the same, in every part, that no 
sensible difference can exist ; and as the tides are 
caused by the differences that occur in the amount of 
attraction, it follows that where there is no difference, 
there is no tide. These remarks apply with greater 
force to the attraction of the sun. It is only in the 
ocean that the expanse of water is sufficiently great 
to cause such an inequality of action, both in the 
lunar and solar attraction, as to produce tides. 

294. In the Mediterranean and Black seas, which 
are almost entirely encircled by land, the tides are 
scarcely perceptible. 



Have tides been noticed in lakes and inland seas ? Why do they Dot occur in 
suoh waters? What is said of the tides in the Mediterranean and Black seas? 



PART THIRD. 

THE STARRY HEAVENS 



CHAPTER I. 

OF THE FIXED STARS IN GENERAL AND THE CONSTELLATIONS. 

295. We pass now in imagination beyond the solar 
system, and direct our attention to those heavenly 
bodies that lie beyond it. 

296. The Fixed Stars. When we gaze at night 
upon the unclouded sky, we behold, in addition to the 
objects already described, a multitude of sparkling 
orbs, varying in brightness and magnitude. These are 
termed the fixed stars, not because they are known to 
be actually stationary in space, for many of the stars 
are undoubtedly in motion, and possibly all may be ; 
but from the fact that their changes in position, 
wherever noticed, are so slow, that, compared with the 
swiftly-moving members of the solar system, they may 
be regarded as fixed. 

297. Magnitudes. Astronomers have classed the 
fixed stars according to their degrees of brightness. 
Those possessing the greatest splendor are termed 
stars of the first magnitude, while others which differ 
from the first by a perceptible diminution of bright- 
ness, rank as stars of the second magnitude; and so on 

What does Part Third treat of? What is the subject of Chapter I. ? To what 
do we now direct our attention ? What is said of the fixed stars ? How have they 
been classed by astronomers? 

7* 



154 ASTRONOMY. 



to the seventh magnitude, which is the limit of 
visibility to \he naked eye. But the telescope now 
comes to our aid, and we discern stars ranging down 
in minuteness from the seventh to the sixteenth mag- 
nitude ; and the series ends, even here, not from the 
want of stars to discover, but because our noblest 
instruments have not sufficient power to detect them. 

298. Number of Stars. The stars are literally 
innumerable. There are but 23 or 24 of the first 
magnitude, from 50 to 60 of the second, about 200 of 
the third ; and as we descend in the scale the number 
comprised in the different classes rapidly increases. 
The number already noted down, from the^Vs^ to the 
seventh magnitude inclusive, amounts to from 12,000 to 
15,000, while the entire number registered amounts 
to 150,000 or 200,000. 

299. But when the telescope sounds the depths of 
space, the heavens appear to be blazing with bright 
orbs, and the more powerful the instrument the more 
numerous are the stars revealed. Sir ¥m. Herschel 
estimated, that, in a certain region of the sky remark- 
ably rich w r ith stars, no less than 116,000 passed 
through the field of his telescope in the space of 
fifteen minutes, and . throughout the entire expanse 
of the heavens, it is reckoned that at least one hun- 
dred millions of stars are within the range of tele- 
scopic vision. 

How many magnitudes are visible to the naked eye ? How far are these mag- 
nitudes extended by the telescope ? What is said respecting the number of the 
stars ? How many are there of the first magnitude ? Of the second ? Of the 
third 7 What is said of their number as we descend in the scale ? How many are 
noted down from the first to the seventh magnitude 1 What is the amount of the 
entire list registered ? What is said of the number of stars observed when the 
telescope is employed ? What estimate was made by Sir William Herscbel ? 



ASTRONOMY. 155 



The Constellations. 

300. In geography r , we observe that the entire surface 
of the globe is divided and subdivided into numerous 
regions and districts, under different names. So like- 
wise in the records of Astronomy we find that, from the 
earliest ages, 1 the visible heavens have been divided 
into spaces termed constellations, which are supposed 
to be occupied by the figures of animals and other 
objects ; and whose names they respectively bear. 

In some few instances the grouping of the stars that 
form a constellation, bears some resemblance to the 
figure which designates it, but for the most part we 
look in vain for any such correspondence. 

301. Their Use. The constellations serve to indi- 
cate in a general manner whereabout a star is situated 
in the heavens, without fixing its exact position. 
Thus if a star is said to be in the head of the Bull, 
we know something respecting its situation, but there 
are many stars in the head of the Bull, and we can- 
not tell what star is meant unless either its right 
ascension and declination are given, or its celestial 
latitude and longitude. These measurements deter- 

1. In the book of Job, which, according to chronologists, was written 
at least 3300 years ago, the constellations of Orion and the Pleiades 
are particularly mentioned. The oldest Greek poets also speak of 
several of the constellations and principal stars. Thus Homer mentions 
Orion, the Bear, the Pleiades, and Hyades. 

How have the visible heavens been divided from the earliest times ? In what 
manner have these spaces been supposed to be occupied? What is said of the resem- 
blance of the grouping of the stars in a constellation to the figure which represents 
it? What do the constellations serve to indicate ? 



156 



ASTRONO M Y. 



mine its precise situation in the heavens, and designate 
the star. 

302. To illustrate from geography. If a traveller 
were to speak of an adventure that occurred in Egypt, 
we should know whereabout on the surface of the 
globe it happened, but not the precise place. This, 
however, we should ascertain at once if the latitude 
and longitude of the place w^ere mentioned. 

303. Principal Constellations. A list of the chief 
constellations is given below. 



CONSTELLATIONS NORTH OF THE ZODIAC. 



Cassiopea, 

Andromeda, 

The Triangles, 

Perseus, 

The Camelopard, 

Auriga, the Charioteer, 

The Lynx, 

The Lesser Lion, 

Ursa Major, the Great 

Bear, 
The Dragon, 
Berenice's Hair, 
The Greyhounds, 
Bootes, 

Mount Menalus, 
Ursa Minor, the Lesser 

Bear, 



The Northern Crown, 
Hercules, 
The Serpent, 
Ophiuchus, 
Lyra, the Harp. 
Aquila, the Eagle, 
Antinous, 
Sobleski's Shield, 
Sagitta, the Arrow, 
The Fox and Goose, 
Cygnus, the Swan, 
Delphinus, the Dolphin, 
The Lesser Horse, 
Pegasus, the "Winged 

Horse, 
The Lizard, 
Cepheus. 



How is the precise situation of a star ascertained ? Illustrate from geography. 
Recite the names of the principal constellations north of the Zodiac. 



ASTRONOMY. 



157 



CONSTELLATIONS OF THE ZODIAC. 



Aries, the Earn, 
Taurus, the Bull, 
Gemini, the Twins, 
Cancer, the Crab, 
Leo, the Lion, 
Yirgo, the Virgin, 
Libra, the Scales, 



Scorpio, the Scorpion, 
Sagittarius, the Archer, 
Capricornus, the Goat, 
Aquarius, the Water 

bearer, 
Pisces, the Fish. 



CONSTELLATIONS SOUTH OF THE ZODIAC. 



The Hydra, 
The Cup, 
Corvus, the Crow, 

The Sextant, 
Centaurus, the Centaur, 
Lupus, the Wolf, 
The Southern Fish 



Cetus, the Whale, 

Eridanus, 

Orion, 

The Hare, 

The Unicorn, 

The Great Dog, 

The Lesser Dog, * 

Argo Navis, the Ship, 

304. Distance of the Fixed Stars. In the years 
1832 and 1833, Professor Henderson, of Edinburgh, 
made an extended series of observations of the most 
refined nature, at the Cape of Good Hope, upon a 
bright star in the constellation of the Centaur, for the 
purpose of ascertaining its distance. He was success- 
ful, and found it to be about twenty millions of 
millions of miles (20,000,000,000,000). Other obser- 
vations made by Mr. McLear in 1839 and 1840, gave 
almost precisely the same result. 

305. The velocity of light is 192,000 miles per 



Recite the names of the principal constellations of the Zodiac, and of those south 
of the Zodiao. What do vre know of the distance of the fixed stars ? 



158 ASTRONOMY. 

second : it would therefore take a ray of light about 
three years and a quarter to travel from the nearest 
fixed star to the earth. 

306. In the year 1838, Professor Bessel, of Konigs- 
berg, ascertained beyond a doubt that the distance 
from the earth to a certain star in the constellation of 
the Swan, was 592,000 times the earth's distance from 
the sun. It would take a ray of light more than nine 
years to pass from this star to our globe. 

307. Nature and Intrinsic Splendor of the Fixed 
Stars. The fixed stars are supposed to be suns 
shining by their own light. The dog-star Sirius, a 
magnificent orb, shines with the brightness of sixty- 
three of our suns. 



CHAPTER II. 

DIFFERENT KINDS OF STARS. STELLAR MOTIONS.— -BINARY 

SYSTEMS, 

308. Periodical Stars. Among the fixed stars, 
several have been noticed which are subject to peri- 
odical fluctuations in "brightness, and in one or two 
instances, the star alternately vanishes and reappears. 
These are termed periodical or variable stars. 

309. Mira. The most remarkable orb of this class, 
and which has been observed for the longest time, is 
the star Mira,m the constellation of the Whale. 

State what is said respecting the nature and intrinsic splendor of the stars 1 
What are periodical or variable stars? 



ASTRONOMY, 159 



Its changing splendor was first noticed by Fabricius, 
in 1596. It appears about twelve times in eleven 
years, shining then for a space of two weeks with its 
greatest brilliancy, sometimes like a star of the second 
magnitude. It then decreases for about three months, 
till it becomes invisible to the naked eye, and so con- 
tinues for the space oi five months more y after which 
it increases vol magnitude and brightness for the re- 
mainder of its period. 

310. Algol. Another conspicuous periodical star 
is Algol, in the constellation of Perseus. 

It generally shines as a star of the second magnitude, 
and continues so for 2d. 13h. 30m., when its splendor, 
all at once diminishes ; and in about 3^ hours it ap- 
pears only as a star of the fourth magnitude. Thus 
it remains for nearly fifteen 7ninutes, when it begins 
to increase, and in 3^ hours regains its original bright- 
ness ; passing through all these variations in 2d. 20h. 
49m. It is the opinion of astronomers, that these 
fluctuations may be caused by the revolution of some 
dark body around this singular star, which intercepts 
a large portion of the stellar light, when it is between 
the star and the earth. Between 30 and 40 variable 
stars have been detected by different observers, whose 
periods of changing brightness vary from &few days 
to many years. 

311. Temporary Stars. In different parts of the 
heavens, stars have now and then been seen shining 
forth with great splendor, and after remaining for 
a while apparently fixed, have gradually faded away, 

Describe the variations of Mira and Algol. What is supposed to be the cause 
of the variations of Algol? How many periodical stars are now known ? What is 
said as to the lengths of their respective period* ? What are temporary stars ? 



160 ASTRONOMY 



and to all appearance become extinct. These are 
called temporary stars, and differ from variable stars 
in this particular, that after once vanishing from our 
sight, they have never been certainly known to reappear 
from time to time. Perhaps when the science of as- 
tronomy is still farther advanced, it may be found 
that temporary stars, so called, are but in fact variable 
stars, of whose long periods of change we are yet 
ignorant. 

312. A temporary star is said to have been ob- 
served by Hipparchus, of Alexandria, in the year 125 
B.C., which suddenly flashed forth in the heavens, 
with such splendor as to be visible in the daytime. 

In the year 389, A.D., a star of this class appeared 
in the constellation of the Eagle. For the space of 
three weeks it shone with the brilliancy of Venus, 
and then died entirely away. Temporary stars, of 
great splendor, were likewise seen in the years 945, 
1264, and 1572, between the constellations of Cepheus 
and Oassiopea. From the circumstance that these 
stars appeared in the same region of the heavens, and 
also from the fact, that the intervals of time between 
their epochs are almost equal, it has been supposed 
that they are one and the same star, which has a 
period of 312 years, or possibly of 156. 

313. The appearance of the star of 1572, was very 
sudden. The renowned Danish astronomer, Tycho 
Brahe, upon returning from his laboratory to his 
house, on the evening of the 11th of November, 1572, 

What may they, perhaps, at length be found to be? Have temporary stars been 
noticed only in modem times? Describe the temporary stars observed in the years 
389, 945, 1264, and 1572, A.D. 



ASTRONOMY. 161 



found a number of persons gazing upon a star, which 
he was confident did not exist half an hour before. It 
was then as brilliant as Sir ins, and continued to in- 
crease in splendor till it exceeded Jupiter in brightness, 
and was even visible at noonday. In December of 
the same year, it began to fade, and by March, 1574, 
had completely disappeared. 

314. Double Stars. Many stars which appear single 
to the unaided eye, are found, when viewed through 
the telescope, to be, in fact, tvjo distinct stars, sepa- 
rated by a very small interval. Moreover, numerous 
telescopic stars, which are seen single, when examined 
with ordinary instruments, are resolved into two, when 
observed through telescopes of high magnifying 
powers. Stars of this kind are termed double stars. 

315. Castor — Alpha Centauri. The bright star 
Castor is one of the finest examples of a double star. 
It consists of two stars, of between the third and fourth 
magnitude, within 5" of each other. Alpha Centauri, 
the nearest fixed star (Art. 304), is also a remarkable 
double star, each of the component stars being, at least, 
of the second magnitude, and separated from each 
other by an interval of about 15". 

316. Colored Double Stars. Many double stars 
display a beautiful variety of colors, the component 
stars being of different hues. Thus, in the case of the 
double star Iota, in the constellation of the Crab, the 
brightest of the component stars is yellow, while the 
other is blue. The double star Gamma, in the con- 
stellation of Andromeda, presents a different variation, 



What are double stars ? Give examples. What peculiarities in respect to the 
colors of the component stars is observed? Give instances of colored double stars. 



162 ASTRONOMY. 



the most brilliant component being red, and its com- 
panion green. 

317. Triple and Quadruple, or Multiple Stars. 
When stars which, under common instruments, appear 
double, are viewed through telescopes of greater power, 
a still further separation is not infrequently effected. 
In some instances, one of the twin stars is resolved 
into two, and the combination is then termed a triple 
star. In other cases, each of the tioo component stars 
is separated into two, and sometimes more ; and since 
all the four appear but as a single star to the naked 
eye, it is called a quadruple, or multiple star. 

FIG. 46. 




THE MULTIPLE STAR THETA. IN THE CONSTELLATION OF ORION. 

318. The number of double stars now discovered, is 
between five and six thousand. 

319. Binary Stars. The double stars are divided 
into two classes. First, those which are optically 
double, 1 the two individuals appearing under ordinary 

1. Thus, in looking over a city, we not unfrequently see two steeples, 
one behind the other, so nearly in the same line of direction that they 
appear as one object. At the first glance the figure formed by their 

What are triple and multiple stars? How many are there? Into how many 
classes are double stars divided? 



ASTRONOMY. 163 



circumstances as one object, simply because they 
happen to be so near to one another that we view 
them in almost exactly the same line of direction. 
J\ T o bond of union exists between them ; for one may 
be millions of millions of 'miles behind the other, and 
altogether beyond the reach of its influence. Secondly, 
double stars which, by their mutual attraction, form 
distinct sidereal systems, the component stars "revolv- 
ing about each other in regular orbits. These, in order 
to distinguish them from double stars in general, are 
termed binary stars? 

320. In 1803, Sir William Herschel first announced 
the fact of the existence of binary stars ; a discovery 
which was the fruit of 25 years' assiduous and close 
observation. At the present time, more than 100 
binary stars have been discovered, and the list is con- 
tinually increasing. 

321. Okbits — Periodic Times. The orbits of 15 
binary stars have been ascertained, and their periodic 
times with more or less certainty determined. Like 
the planets of our system, they revolve in elliptical 
paths. Their known periodic times range from 31 to 
736 years. 

union may seem single; a closer inspection shows that it is optically 
double. 

1. Binary, from the Latin binus, meaning two and two, by couples. 



Describe these classes. When and by whom was the existence of binary stars first 
announced? Of how many years' research was this discovery the fruit? How 
many binary stars are at present known? Of how many have the orbits and 
periodic times been ascertained with more or less accuracy? Jn what kind of orbits 
do they revolve? What is said of the extent of their periodic times? 



164 ASTRONOMY. 



CHAPTEE III. 

STARRY CLUSTERS NEBULA NEBULOUS STARS ZODI- 
ACAL LIGHT MAGELLAN CLOUDS STRUCTURE OF THE 

HEAVENS. 

322. Starry Clusters. "When we turn our gaze 
upon the heavens in a serene night, we perceive that 
in some parts the stars are more crowded together than 
in others, forming, by their close proximity, groups or 
clusters. Such a cluster is the Pleiades, in which six 
or seven stars are seen by the naked eye, but where 
the telescope reveals fifty or sixty comprised within a 
very small space. 

323. The central portion of a cluster is usually most 
thickly sown with stars, and the stellar light there 
shines forth with the greatest brilliancy. A beautiful 
cluster of this kind is found in the constellation of 
Hercules. It is represented in Fig. 47. 

324. The stars that compose a cluster are often ex- 
ceedingly numerous. It has been estimated that not 
less than five thousand stars exist in some of the 
groups, wedged together into a space in the heavens, 
the area of which does not exceed one tenth part of 
that covered by the moon. 

325. Milky Way, or Galaxy. 1 The most mag- 

1. From the Greek word gala, signifying milk. 

Describe some of the stellar clusters. What is the usual appearance presented by 
the central portion of a cluster? What is said respecting the number of stars they 
contain? 



ASTRONOMY. 165 



FIG. 47. 




A CLUSTER OF STARS IN HERCULE 



nificent stellar cluster, by far, is the milky ivay r which 
like a broad zone of light encompasses the heavens. 
Its brightness is derived from the diffused radiance 
of myriads of myriads of stars that compose it, whose 
splendors are blended together into a milky whiteness 
on account of their immense distance from us. 

326. In this cluster, Sir William Herschel estimated 
that, during one hour's observation with his telescope, 
no less than 50,000 stars passed before his sight with- 
in a zone 2° in breadth. Sir John Herschel has 
computed that the number of stars in the milky way, 
sufficiently visible to be counted, when viewed with 
his 20 feet telescope, amount in both hemispheres to 



Which is the most splendid stellar cluster that the heavens present? What is 
its aspect, and whence is its light derived ? What observations and computations 
have been made which show that it contains a vast number of stars ? 



166 ASTRONOMY. 



five and a half millions. The actual number in this 
cluster he considers to be much greater, since in some 
parts they are so crowded together as to defy enu- 
meration. Our sun is supposed to be one of the stars 
belonging to this group. 

Nebulje. 

327. Scattered throughout the sky are seen, either 
by the naked eye or by the aid of the telescope, dim, 
misty objects, of various shapes and sizes, stationary to 
all appearance, like the stars themselves. These 
objects are named nebulce. 1 One of the finest is 
situated in the girdle of Andromeda. It is visible 
to the naked eye, and was noticed and described by 

FIG. 48 



NEBULA OF ANDROMEDA. 



1. For the meaning of this word, see page 9, note 3. 

What is supposed of our sun ? What are nebulae? Which is one of the finesi * 



ASTRONOMY. 167 



Simon Marius in 1612 ; and there is reason for believ- 
ing that it was seen even as early as 995. This 
nebula is of vast size, extending over an area 15' in 
diameter. It is delineated in Fig. 48. 

328. Another splendid nebula is in the sword- 
handle of Orion. It consists of straggling, cloudlike 
spots, occupying a space in the heavens considerably 
larger than the disk of the moon. This nebula was 
discovered by Huvghens in 1656. In Fig. 49, its 

FIG. 49. 




NEBULA OF ORION. 



What is said of the neoula in Orion ? 



168 ASTRONOMY. 



central portions are shown as they have been delin- 
eated by Sir John Herschel. 

329. Their Constitution. Stellar clusters and neb- 
idee have usually been regarded as distinct classes of 
celestial objects ; the former consisting of groups of 
stars, either visible to the naked eye or through the 
telescope, and the latter of vast collections of un- 
formed matter diffused through the infinitude of space. 
But it is by no means certain that such a distinction 
exists in nature, for the late discoveries of eminent 
astronomers point to the conclusion, that the nehulce 
are clusters of stars more or less distinct. 

330. Number and Distance of Stellar Clusters 
and Nebulae. About two thousand stellar clusters 
and nebulce were observed by Sir William Herschel. 
In 1833, the list amounted to two thousand five hun- 
dred, and this number was increased to about four 
thousand by the splendid discoveries made by Sir 
John Herschel, during his residence at the Cape of 
Good Hope. The distance of the nebulas from the 
earth is vast beyond conception. The ring nebula in 
the constellation of the Lyre is so remote, that astron- 
omers assert a ray of light cannot reach us from this 
object in less than twenty or thirty thousand years. 

The nebula of Orion is still more distant, for it is 
computed that a ray of light, moving as it does with 
a velocity of 192,000 miles a second, would occupy 
not less than sixty thousand years in travelling from 
this nebula to the earth. 



What views have been entertained respecting stellar clusters and nebulae? Re- 
late in full what is said respecting the number and distance of stellar clusters and 
nebulce. State what is said of our distance from the nebula of Orion. 



ASTRONOMY. 169 



331. Their Physical Structure. Mathematicians 
have clearly shown it to be utterly impossible that 
the stars composing individual clusters and nebulae 
could have been so grouped together by mere chance. 1 
Their union must consequently be the result of some 
physical law impressed upon them by their Creator, 
in virtue of which they are combined in harmonious 
systems. 

332. Nebulous Stars. In various parts of the 
heavens, bright and sharply-defined stars are beheld 
enveloped in a cloud-like disk or atmosphere. These are 
called nebulous stars. Such a star is shown in Fig. 50. 

PIG. 50. 




A NEBULOUS STAR. 



333. Zodiacal Light. The zodiacal light is a 
luminous object shaped like a pyramid, that accom- 
panies the sun in his apparent course through the 
heavens. 

1. Mitchell has shown that if 1500 stars, like the six brightest in the 
Pleiades, were scattered at random through the heavens, there would be 
only one chance out office hundred thousand that any six of them would 
come as close together as they do in the Pleiades. 



What is remarked in regard to the physical structure of stellar clusters and neb 
ulae ? What is a nebulous star ? What is the zodiacal light ? 



170 ASTRONOMY. 

334. Aspects. According to Professor Olmsted, 
who has made this phenomenon an especial study, 
the zodiacal light, in our climate, becomes visible in 
the eastern sky about the beginning of October. It is 
then seen before the dawn, its base resting upon that 
part of the horizon where the sun at this time rises, 
the luminous pyramid extending obliquely upward, 
until ita point reaches above the starry cluster of the 
Beehive, in the constellation of Cancer. Throughout 
the month of December it is beheld on both sides of 
the sun, being seen in the morning before sunrise, 
and in the evening after sunset, extending in the frst 
case sometimes as far as 50° westward from the sun, 
and in the second 70° eastward. In February and 
March, the zodiacal light appears only in the west, 
after sunset; it is then most conspicuous, and its 
luminous point is seen as far up as the Pleiades. 

335. Size. This object possesses no well-defined 
outline, but its light gradually fades away from the 
central to the outer portion, until it becomes too faint 
to be discerned. Its breadth at the base varies from 
8° to 30° according to Herschel, but Professor Olm- 
sted has noticed it when it was 40° in width. 

From the observations of the latter gentleman it 
appears, that the zodiacal light sometimes extends in 
length considerably beyond the orbit of the earth ; for 
on the 18th of December, 1837, it was beheld stretch- 
ing away eastward from the sun to the distance of 
120°. 

336. Nature. Sir John Herschel conjectures that 

What are its aspects in our climate ? What is stated respecting the size of tho 
zodiacal light ? 



ASTRONOMY. 171 



the zodiacal light is an elongated oval-shaped, envelope, 
enclosing the sun, consisting of extremely light mat- 
ter, and possibly composed to a great extent of the 
same materials which form the tails of comets. Under 
this view, the sun, surrounded by the zodiacal light, 
presents a phenomenon similar to that of the nebulous 
stars. 

337. Structure of the Heavens. Different systems 
have from age to age been presented to the world, 
professing to explain the structure of the heavens. 
The three which especially deserve notice are the 
Ptolemaic, the Tychonic, and the Copernican. 

338. Ptolemaic. According to this system the 
earth is immovably fixed in the centre of the universe, 
while all the heavenly bodies revolve about it from 
east to west. It was established by Ptolemy, an 
Egyptian astronomer, in the second century of the 
Christian era, and prevailed for more than 1500 
years. 

339. Tychonic. This system originated with Tycho 
Brahe, who flourished in the sixteenth century, Like 
Ptolemy, he believed that the earth was stationary in 
the centre of the universe, and that the stars, and the 
Bun and moon, revolved around it ; but he conceived 
that the planets revolved directly about the sun. 

340. Copernican. So called from Nicholas Coper- 
nicus, an illustrious astronomer of the fifteenth cen- 
tury. According to the Copernican system, the earth 
rotates on her axis from west to east, and revolves with 

What are the views of Sir John Herschel in regard to its nature? What are those 
of Professor Olmsted ? Give an account of the principal different systems which 
have professed to explain the structure of the heavens. 



172 ASTRONOMY 



the rest of the planets around this sun, in the same 
direction.- This system is the true one, for it is not 
only mathematically demonstrated to be correct, but 
it perfectly explains all celestial phenomena, which 
every other system fails to do. 

341. The structure of t the heavens was briefly ex- 
plained in the beginning of this work (Arts. 3, 4), in 
accordance with the Copernican system, and as we 
have advanced in our investigations, it has been 
gradually unfolding in part to our view. 

Commencing with the earth, we have found that it 
both rotates on its axis and revolves about the sun, 
while around it circles a shining moon. It has been 
further shown that the earth is not isolated, but is 
one of a brotherhood of planets, endowed with the 
same motions, and in several cases similarly attended. 
All these, with myriads of comets, constitute the solar 
system, 

342. Exploring further, we behold in the binary 
stars, suns revolving about each other, with their 
respective trains of planets and comets, exhibiting the 
phenomenon of solar systems in motion. 

343. Piercing deeper into abysses of space, stellar 
clusters and nebulae stand forth revealed, objects of 
surpassing grandeur and magnificence. For here 
suns crowd upon sims, forming a vast and numerous 
group of solar systems — united, to all appearance, by 
a common bond. Possibly these associated systems 
revolve about some mighty sun centrally situated 
within the radiant group; for if our solar system, 



Which is the true system, and why ? Explain the structure of the heavens in 
accordance with the Copernican system. 



ASTRONOMY. 173 



together with, the stars that glitter in our firmament, 
is really revolving around some central sun, as some 
astronomers have supposed, analogy would lead us to 
infer, that similar motions also exist amid these starry 
clusters and nebulae. *, 

344. When the scroll of the skies is still farther 
unrolled for our perusal, we may perhaps find that 
these island universes themselves move in orbits 
around some common centre. 

For with all our surprising discoveries, we are yet 
upon the very threshold of creation ; and could we 
continue to explore beyond the remotest nebulae, 
through the successive realms of space, new scenes of 
grandeur would perpetually unfold; and new fields 
of Omniscient display would be constantly revealing 
that God was still before us in his creative energy, 
and that we saw but the " hidings of his power." 



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W. W. Benjamin, N. H A. Holmes, K Y. 



Dr. Bullions* Series of Works. 



Jas. E. Lattimer, K H. E. J. Avery, Mass. 

John Trembly, Ohio. Prof. H. Wheeler, Ind. 

F. Crafts, Mass. Prof. J, Towler, 1ST. Y. 

C. Walker, Mass. W. L. Nicholas, Ohio. 

A. Smyth, Ohio. Rev. George Loomis, Del 
W. B. Bunnell, H> Y. J. Reid, R. I. 

R. D. Van Kleck, R. I. W. E. Todd, 1ST. H. 

Jas. J. Helm, IN". J. R. W. Finley, Mo. 

Jos. J. Fravelli, Pa. Robert Thomson, K Y. 

R. H. Bishop, Ohio. David Parsons, Ohio. 

B. C. Ward, Pa. Isaac Booth, Pa. 

O. II. Drummond, Ohio. ' J. A. Goodwin, K Y. 

Rev. J. McCanley, Va. O. L. Leonard, Ky. 

Wm. M. Russell, Mass. E. C. Boyle, Ohio. 

D. Harris, N. J. M. H. Patten, Mo. 
J. P. W. Jenks, Mass. A. C. Roe, Conn. 
A. Mong, Pa. Lewis Bradley, Pa. 
Joel Whitney, 1ST. Y. Charles A. Lord, Mo. 
Lewis Vail, Pa. Rev. A. A. Livermore, 2ST. H. 

E. L. Hazeltine, D. D., S. C, Pliny Fisk, N. Y. 

E. D. McMaster, Ohio. Lyman Harding, Ohio. 

L. Strong, N. Y. Wm. Jones, Oregon. 
And others from all parts of the Union. 



From the Southern Repertory and College Revieic. 

" Bullions' Series of Grammars and Elementary Classics, through the 
kindness of the publishers, have been placed upon our table. Although 
we have been familiar with some of Bullions' books for years, we have 
not had until now the opportunity of examining the entire series of 
grammars. This examination we have made with much pleasure, which 
increased as we progressed. We think that these books ought to be in- 
troduced into our primary schools, academies, and colleges forthwith, 
even to the exclusion of others, which were good ' in their day,' but 
which have got behind the times. Every teacher has experienced the 
inconvenience, and every student has felt the embarrassment, arising 
from a change of text-books from one author to another, on the same 
subject. True, principles may be ever the same; but each author has 
his own mode of expression and illustration — each has his plan. 

" In the series one uniform plan is pursued through the grammars 
of the English, Latin, and Greek languages. The young beginner who 
masters the Practical Lessons in English Grammar, is not only prepared 
for the Analytical and Practical Grammar, and the still higher Exercises 
in Analysis and Parsing, but can take hold of the Latin and Greek 
Grammars, with a good knowledge beforehand of the plan to be pur- 
sued. We are for Bullions' books, as well as for some others, which we 
hope hereafter to notice, issued from the press of those excellent book- 
makers, Farmer, Brace & Co., late Pratt, Woodford & Co." 



Cooper 1 s Virgil. 9 



I nse Bullions' works — all of them — and consider them the best 
of the kind that have been issued in this or any other language. 
If they were universally used we would not have so many super- 
ficial scholars, and the study of the classics would be more likely 
to serve the end for which it was designed — the strengthening and 
adorning of the mind. J. B. Thompson, A. M., 

late Rector of the Somerville Classical Institute, JV". J. 

Dr Bullions' English Grammars are introduced into the Public and 
many of the Private Schools, the Latin School, the English High School, 
the City Normal School, of the city of Boston ; Normal Schools of 
Biidgewater and Westrield ; Marlborough Academy ; cities of Salem 
Newbuiyport, &c , Mass. ; Portsmouth, and several academies in New 
Hampshire ; and re-adopted in Albany and Trey, New York. They are 
used in over seventy academies in New York, and in many of the most 
nourishing institutions in every State of rhe Union. 



Cooper's Virgil, with English Notes.— (2 00.) 

Having examined the Rev. J. G. Cooper's edition of the works 
of Virgil, I have no hesitation of giving my opinion, that the plan 
which he has pursued is excellent, and the execution highly credit- 
able to his talents and scholarship. Such a work will greatly 
facilitate the study of the poet, on the part of the youthful learner. 
It will give him a correct idea of the meaning of the author in the 
more difficult passages, and by its copious notes upon ancient his- 
tory and mythology, will enable him to relish beauties that are 
now rarely perceived in the early course of classical instruction. I 
have no doubt but that its appearance will be welcomed by the in- 
telligent and discerning, as a publication admirably adapted to en- 
list the feelings and stimulate the application of youth, in the 
elementary schools of our country. 

Geoege P. Chapman, D. D., 
formerly of Pennsylvania University. 

Similar opinions have been expressed by the following literary 
and scientific gentlemen : 

James Ross, LL. D., John T. Kirkland, D. D., 

James Renwick, LL. D., Henry Ware, D. £)., 

W. C. Wyatt, D. D., John S. J. Gardner, D. D., 

William Harris, D. D., Wm. Rafferty, D. D., 

John Bowden, D. D., Edward Sparks, M. I)., 

James Kemp, D. D., E. D. Barry, D. D., 

Gideon Blackburn, D. D., Prof. J. S. Kingsley, Yale Coi. 
And many others. 

1* 



10 Prof. Peissnerh English- Gerrnan Grammar. 

A Comparative English-German Grammar.— ($1 oo.) 
Based on the affinity of the two languages, by Peof. Elias Peissnee, 
late of the University of Munich, now of Union College^ 
Schenectady. 

From the New YorJo Churchman, 

Of all the German Grammars we have ever examined, this is 
the most modest and unpretending — and yet it contains a system 
and a principle which is the life of it, as clear, as practical, as effec- 
tive for learning Grammar as any thing we have ever seen put 
-forth, with so much more pretence of originality and show of phi- 
losophy. In travelling from England to Germany, a man might 
commence his journey in England : he must first pass through 
those parts which lie most contiguous to the land to which he is 
going ; he should cross the separating line at the point or points 
where the two adjacent countries have most features in common, 
and his first explorations in the new land will be in those quarters 
which remind him most of the scenes and associations from which 
he is departing. This is the pervading principle of the Grammar 
before us, and, truism as it may appear, it contains the secret of 
the easy acquisition of a foreign language, especially one that has 
many affinities both in words and idioms to our own. 

The principle on which this book is grounded gives it a strong 
claim to every teacher through examination. It will be found, 
too, we think, that the author has not only presented a new idea 
of much interest in itself, but has admirably carried it out in the 
practical lessons and exercises of his work. 

From Professor J. Foster, of Schenectady. 
I have examined Prof. Peissners German Grammar with some 
attention ; have marked with interest the rapid advancement of 
students here using it as a text-book, and have myself carefully 
tested it in the instruction of a daughter eleven years of age. The 
result is a conviction that it is most admirably adapted to secure 
easy, pleasant and real progress, and that from no other work 
which has come under my notice can so satisfactory a knowledge 
of the language be obtained in a given time. 

From the Albany Morning Express. 

This is one of the very best treatises of its kind now extant. 
Those who are acquainted with the science and practice of lan- 
guage will need but a simple statement of some of its points, in 
order to appreciate its superior merit. 

From the Schenectady Reflector. 
It seems to us to meet more successfully than any other Gram 
mar, the case of those who desire an accurate knowledge and prac- 
tice of the German language, through a method at once easy, rapid, 
and scientific. 



Prof, j; B. Dodd's Series. 1 1 



PROF. J. B. DODD'S MATHEMATICAL SERIES 

COMPEISES — 

An Elementary and Practical Arithmetic, . $0 45 

High School Arithmetic, ..... 84 

Elements of Algebra, 84 

Higher Algebra, 1 50 

Key to Algebra, ......... 84 

Elements of Geometry, 1 00 

AIND KEYS TO THE ABOVE. 

These Arithmetics are believed to be unrivalled in the follow- 
ing particulars : 

1. Yh.% philosophical ace urateness with which their topics are 
arranged so as to show the mutual dependence and relationship of 
their subjects. 

2. The scientific correctness and practical convenience of their 
greatly improved nomenclature. 

3. The clear and concise manner in which principles are stated 
and explanations are given. 

4. Brevity and completeness of rules. 

5. The distinctness with which the true connection between 
Arithmetic and its cognate branches is developed. 

6. The excellent and thorough intellectual discipline super 
induced. 



RECOMMEND ATIOisTS. 

From R. T. P. Allen, Superintendent of Kentucky Military 

Institute. 

" Upon a careful examination of a manuscript Treatise on 
Arithmetic by Prof. Dodd, I find it greatly superior to all others 
which have come under my notice, in system, completeness and 
nomenclature. The arrangement is natural, the system complete, 
and the nomenclature greatly improved. These improvements are 
not slight ; they are fundamental — eminently worthy the attention 
of the Mathematical Teacher, and give a character of unity to the 
work, which at once distinguishes it from all others on this subject. 

" I believe it admirably adapted to the purposes of instruction ; 
in fact, by far the most convenient and usable booh for teacher and 
pupil I have yet met with; and will, with great pleasure, adopt it 
in the Institute, and recommend its adoption by all." 

From John BrocMesby, A. M., Prof of Mathematics and Natural 
Philosophy, in Trinity College, Conn. 
" From a careful examination of the Arithmetic of Prof. Dodd, 
I have been led to entertain a favorable opinion of the work. It is 



12 Prof. J. B. Dodd's Series. 

philosophical in its arrangement, and exact and clear in its rnles 
and explanations. The examples are such as to bring the mind of 
the pupil into active exercise. I should select this book to place 
in the hands of a child in preference to others upon the same sub- 
ject which have obtained a wide-spread circulation." 

From Jemiette L. Douglass, Newburgh, N. Y. 

"I have examined with great care, and much interest, * Dodd's High 
School Arithmetic,' and am fully convinced that it is a book of rare 
merit, and is not surpassed by any in use. Furthermore, after a year's 
careful study of ' Dodd's High School Algebra,' and after having thor- 
oughly tested the application of its rules, and the precision, clearness and 
force of the same, hesitate not to say that it has no superior, if any equal, 
in the range of Algebraic Science." 

From the Associate Principal of Mount Palatine Academy. 

" I have examined Dodd's Arithmetic, and am fully persuaded 
that it is superior to any other with which I am acquainted. I 
could speak in detail were it necessary ; but all that is required 
to establish its reputation and introduction, is to have it known by 
Teachers" 0. M. Weight, Inst. 

From the Faculty of Rock River Seminary. 
" Upon an examination of Prof. Dodd's Arithmetic, we have 
come to the conclusion that its superior arrangement, the clearness 
of its rules and explanations, and its improved nomenclature, entitle 
it to the careful consideration of the Mathematical Teacher: that 
these improvements distinguish it from all others that have come 
under our notice. We therefore give it our cordial approbation, 
and shall introduce it immediately into our Seminary." 

D. J. Pinoxney, Principal. 

S. M. Fellows, Prof, of Mathematics. 

Silas Seaele, Prof, of Languages. 

" Dodd's High School Arithmetic is better adapted to supply the 
wants of High Schools and Academies than any other Arithmetic 
which I have ever met. His rules are plain, concise, definitely 
stated, and fully illustrated with examples." 

GL M. Baekee, 
Baldwin Institute, Ohio. 

" I have Dodd's Higher Arithmetic, and unhesitatingly pro- 
nounce it the best work for advanced classes I have ever seen." 

M. S. LlTTLEFIELD, 

Grand Rapids, Mich. 



Prof. J. B. Dodd's Series. 13 

Similar testimonials have been received from the following 
gentlemen whose names are attached, in favor of one or both these 
Arithmetics : r 

H. A. Wilson, Jonesville. L. Dickerman, N". H. 

R. S. Thurmer, Ind. J. G. Hoyt, do. 

A. P. Chute, Mass. M. L. Brown, ri. Y. 

Rev. J. A MeCanlev, Va. W. E. Pierce, Ohio. 

W. Spindler, Ohio. " K T. Allen, Mass. 

J. W. P. Jenks, Mass. N". McDougall, N. Y. 

Rev. W. L. Harris, Wesleyan A. Wood, Maine. 

University, Ohio. J. R. Art, Indiana. 

A. K. Slade, Mass. A. Morse, Nantucket. 

W. P. Clark, Mich. G. C. Merrifield, Ind. 

James Campbell, Ohio. T. A. Benton, Ohio. 

W. W. Howard, Ky. Isaiah Dole, Maine. 

W. B. Slaughter, Pa. J. Estabrook, Mich. 

W. A. Bacon, Mich. J. Towler, N. Y 

Rev. George Loomis, Delaware. M. F. Cowdery, Sandusky, Ohio. 
C. B. Crump, K Y. 

Prof. Dodd's Algebras have received the most flattering enco- 
miums from teachers who have used them in the school-room. 
They are, probably, the most clear and comprehensive works on 
Algebra in print. 



TESTIMONIALS. 

We have introduced Dodd's Algebra into the Genesee Wes- 
leyan Seminary, as a permanent text-book. 

Prof. W. H. De Put. 

Dodd's Algebra possesses excellencies pertaining to no other 
work. E. H. Mooee, 111. 

I am much pleased with Dodd's Algebra, and will introduce it. 

Eev. J. A. McOanley, Va, 

I use Professor Dodd's Algebra, and shall continue it as our 
regular text-book. Oscae Haeeis, F. J. 

From Peof. A. L. Hamilton, Prest. of Andrew College. 

I have examined with some care Prof. Dodd's Elements of Geo- 
metry, and, so far as I am capable of judging, I conceive it to be 
in many respects decidedly the best work of the kind extant. For 
simplicity, exactness, and completeness, it can have no superior. 
Like his Arithmetic and Algebnj, in many important particulars, 
his Geometry stands pre-eminent, and alone. 



14 Enos^s Intellectual and Practical Arithmetic. 

Scliell's Introductory Lessons in Arithmetic,— ($1 oo.) 

Designed as an introduction to the study of any mental or written 
Arithmetic. It contains a large amount of mental questions, toge- 
ther with a large number of questions to be performed on the 
slate ; thus combining' mental and written exercises for young be- 
ginners. This is a very attractive little book, superior to any of 
its class. It leads the pupil on by the easiest steps possible, and 
yet insures constant progress. 



RECOMMENDATION'S. 

I have carefully examined the manuscript of ct Schell's Intro- 
ductory Lessons in Arithmetic," and am convinced that it is alto- 
gether superior to any text-book of the kind with which I am ac- 
quainted. It is peculiarly adapted to the wants of beginners, the 
language being simple, the definitions clear, the examples easy, and 
the transition from one subject to another gradual and natural. 
I cannot too much commend the system which the author has 
adopted throughout, of fully illustrating every principle as he ad- 
vances, by numerous mental and written exercises, rendering 
thereby one rule perfectly familiar before he passes to the next. 

It is unnecessary to do more than to ask the attention of teachers 
to this work ; they cannot examine it impartially without being 
convinced of its superior merits. It will, no doubt, become one 
of the most popular of school-books. 

Geo. Pay^e Quackenbos, 
Hector of Henry st. Grammar School, ffl. T. 

I wish to introduce Schell's little Arithmetic. It is just the 
thing for beginners. Send six dozen. J. Maekham, Ohio. 

I am highly pleased with Schell's little book, and shall use it. 

G. G. Meeeifield, Ind. 

Schell's little book for children is a lean-ideal of my own, and 
of course it suits. D. F. Dewolf, Ohio. 

The School Committee have adopted Schell's Arithmetic for 
our public schools. Send us three hundred. 

D. G. Heffeon, Supt. Schools, Utica. 



An Intellectual and Practical Arithmetic— (So 25); 

Or, First Lessons in Arithmetical Analysis. By J. L. Enos, 
Graduate of the N". Y. Slate Normal Schools. 

The same clearness and conciseness characterize this admirable 
book that belong to the works of Prof. Dodd. The natural ar- 
rangements of the text, and the logical mode of solving the ques 



Whitlock ) s Geometry and Surveying. 1 5 

tions, is a peculiar and important feature belonging to this book 
alone. 



RECOMMENDATIONS. 

I have examined with care and interest, Enos's Mental Arith- 
metic and shall introduce it at once into the Academy. 

Prof. 0. K Weight. 

We have examined an intellectual Arithmetic, by J. L. Enos, 
and like it much. We shall immediately use it in our school. 

Prof. D. I. PlXCKNEY, 

S. M. Fellows, 
S. Seaele, 

Rock River Seminary. 

Having used Enos's Mental Arithmetic in my school, I believe 
it to be superior to all other works of the kind. 

W. Bailey, K Y. 



Whitlock's Geometry and Surveying,— ($i 50.) 

Is a work for advanced students possessing the highest claims upon 
the attention of Mathematical Teachers. In comparison with other 
works of the kind, it presents the following advantages : 

1. A better connected, and more progressive methed of geo- 
metrizing, calculated to enable the student to go alone. 

2. A fuller, more varied and available practice, by the intro- 
duction of more than four hundred exercises, arithmetical, demon- 
strative and algebraical, so chosen as to be serviceable rather than 
amusing, and so arranged as greatly to aid in the acquisition of the 
theory. 

3. The bringing together of such a body of geometrical know- 
ledge, theoretical and practical, as every individual on entering into 
active lite demands. 

4. A system of surveying which saxes tico thirds the labor re- 
quired by the ordinary process. 

This work is well spoken of universally, and is already in use in 
some of the best institutions of this country. It is recommended 
by Prof. Pierce, of Cambridge ; Prof. Smith, of Middletown ; Prof. 
Dodd, of Lexington, and many other eminent mathematicians. 

From E. M. Moese, Esq : — I consider that I have obtained more 
mathematical knowledge from Whitlock's Geometry, than from all 
other text-books Combined. Unlike too many treatises of a similar 
nature, it is eminently calculated to make mathematicians. 



16 



Dr. Comstoc&s Series. 



Prof. Palmer's Bookkeeping,— {$o 67.) 

KEY AND BLANKS. 

This excellent book is superior to the books generally used 
because 

1. It contains a large number of business blanks to be filled by 
the learner, such as deeds, mortgages, agreements, assignments, 
&c. &c. 

2. Explanations from page to page, from Article to Article, and 
to settle principles of law in relation to deeds, mortgages, &c, &c. 

3. The exercises are to be written, out, after being calculated. 
In other works the pupil is expected to copy, merely. 



EEOOMMENDATIONS. 
Joseph H. Palmee, Esq. ; 

Sir,— It has afforded me pleasure to read your excellent 
Treatise on Bookkeeping. The Perspicuity of its style is admirable, 
and with its peculiar arrangement, with references and laconic defi- 
nitions, makes it at once invaluable to the young accountant, as a 
primary and practical work on the most approved method of keeping 
accounts. Hiram Dixon, 

Accountant at Adams & Go., 16, 18, 19, Wall st. 

Similar testimonials have been received from the teachers of the 
Free Academy, and others, 1ST. Y. 



Horace Webster, LL. D. 

J. J. Owen, D. D. 

G. B. Docharty, LL. D. 

J. T. Bentdice, A. M. 

J. Graef Barton, A. M. 

D. Cartledge. 

A. H. Wheeler. 

Wm. Palmer. 

D. K. Bull. 

S. Kendall. 

Joseph Keen, Super'nt Com. 

Schools, N. Y. City. 
J. J. Doane, Principal Ward 

School No. 20, N. Y. 
Thos. Faulke, Principal Ward 

No. 30, N. Y. 



N. W. Starr, Principal Ward 

School No. 29, N. Y. 
J. E. Whitehead, Principal 

Ward School No. 23, N. Y. 
J. J. Anderson, Principal Ward 

School No. 16, N. Y. 
L. Hazeltine, Principal Ward 

School No. 14, N. Y. 
S. Reynold, Principal Wil- 

liamsburgh Grammar School. 

N. Y. 
A. Marceilus, Principal Wil- 

liamsburgh Academy. 
H, I). Wood worth, Principal 

Ward School No. 2, N. Y. 



Dr. Comstock's Series of Books on the Sciences, viz. : 

Introduction to Natural Philosophy. For Children. . $0 42 

System of Natural Philosophy, Newly revised and enlarged, 

including late discoveries, . . . . . 1 00 



Dr. Comstoc&s Series. 



17 



Elements of Chemistry. Adapted to the present state of the 

Science, .......... 1 00 

The Young Botanist. New edition, . . . . 50 

Elements of Botany. Including Vegetable Physiology, and a 

Description of Common Plants. With Cuts, * . . 1 25 

Outlines of Physiology, both Comparative and Human. To 
which is added OUTLINES OF ANATOMY, excellent for 
the general scholar and ladies' schools, . . . . 80 



1 25 

ns 



50 

50 

1 00 
30 



New Elements of Geology, Highly Illustrated, 
Elements of Mineralogy. Illustrated with numerous Cuts, 
Natural History of Birds. Showing their Comparative Size. 

A new and valuable feature, 

Natural History of Beasts. Ditto. 

Natural History of Birds and Beasts. Ditto. Cloth, 
Questions and Illustrations to the Philosophy, . 

All the above works are fully illustrated by elegant cuts. 
The Philosophy ha3 been republished in Scotland, and trans- 
lated for the use of schools in Prussia. The many valuable addi- 
tions to the work by its transatlantic Editors, Prof. Lees, of 
Edinburgh, and Prof. Hoblyn, of Oxford, have been embraced by 
the author in his last revision. The Chemistry has been entirely 
revised, and contains all the late discoveries, together with methods 
of analyzing minerals and metals. Portions of the series are in 
course of publication in London. Such testimony, in addition to 
the general good testimony of teachers in this country, is sufficient 
to warrant us in saying that no works on similar subjects can equal 
them, or have ever been so extensively used. It is a remarkable 
fact, that when interested persons have attacked these works, and 
succeeded in getting in their own, a little time has dissipated the 
mist, and they have found their way hack again, A new edition 
of the Botany, with an enlarged Flora, is just ready. 

The Phylosophy has just been thoroughly revised, andfor the sixth time 
newly stereotyped. There is no book of its size in the world (we believed 
that has ever had a circulation equal to it. 

RECOMMENDATIONS. 



John Griscom, LL. D., K Y. 

W. H. Seward. 

W. T. Bonte, Canada. 

R. M. Brown, N. Y. 

A Wood, N. J. 

M. P. Covert, N. J. 

B. Hallowell, Ya. 

A. L. Smith, Ya. 

A. PL Drummond, Ohio. 



A. C. Wright, D. C. 
A. McDougald, N* Y. 
G. C. Merrifield, Ind. 
Rev. J. P. Cowles, Mass. 
M. E. Dunham, K Y. 
J. M. Stone, IS T . H. 
W. R. White, Ya. 
A. F. Ross, H". Y. 
T. Yalentine, Albany. 



The sale of 800,000 copies of the Philosophy would seem to 
render notices superfluous. 



18 Prof. Hookers Physiology. 

Human Physiology and Kygeine— ($1 25.) First Ecck in do— ($o 66) 
Designed for Colleges and the Higher Classes in Schools, and for 
General Reading. ByWorthistgton Hooker, M. D., Professor 
of the Theory and Practice of Medicine in Yale College. 
Illustrated with nearly 200 Engravings. 
This is an original work and not a compilation. It presents the 
subject in a new light, and at the same time embraces all that is 
valuable for its purpose, that could be drawn from the most emi- 
nent sources. The highest encomiums are received from all quar- 
ters ; a few are subjoined. 

"We can truly say that we believe this volume is of great value, 
and we hope that the rare merits of the diligent author will be 
both appreciated and patronized. 

Boston Medical and Surgical Journal. 

Dr. Hooker writes with perspicuity, explains difficult points 
with simplicity, and adapts the subject well to school instruction 
and general reading. American Journal of Science and Arts. 

Here is the remedy for a want which is so evidently a want, 
and that now we have it supplied, it seems an absurdity to have 
lived on wanting it. The present work is a popular treatise, at- 
tractive enough to be read, and with compass enough to allow the 
author's fertility of illustrative anecdote to come into play. There 
is no need of commending the work to the attention of a com- 
munity where Dr. Hooker is so well knows, as he is among us. 

Norwich Courier. 

I am ready to pronounce it unqualifiedly the most admirable 
book or work on the human system that has fallen under my 
notice, and they have not been few. If any one desires a complete 
and thorough elucidation of the great science discussed, they can 
nowhere be better satisfied than in the perusal of Dr. Hooker's 
most excellent work. B. F. Tewksbury, Lenoxville, Pa. 



" I have thoroughly examined Hooker's Physiology. The hints con- 
tained ia the Preface are of more value to a practical teacher than many 
entire works upon the same subject. In my opinion the book needs only 
to be known to be appreciated, it will speak for itself. 

E. F. Stkong, Principal Public High School, West Meriden, Ct. 



Elements of Meteorology.— (|o 75.) 

Designed for Schools and Academies. By John Bjrockelsby, A.M., 

Professor of Mathematics and Natural Philosophy in Trinity 

College, Hartford. 

The subject of Meteorology is of the deepest interest to all. Its 

phenomena every where surround us, and ought to be as familiarly 



Prof. Brocklesby^s Series. 19 

known by the scholar as his arithmetic or philosophy. This work 
treats on " Winds in General, Hurricanes, Tornadoes, Water Spouts, 
Kain, Fogs, Clouds, Dew, Snow, Hail, Thunder-storms, Rainbow, 
Haloes, Meteorites, Northern Lights, &c. 

TESTIMONIALS. 

From Denison Olmsted, LL. D., President of Yale College. 

" I have perused your work on Meteorology, which you were 
so kind as to send me, and am much pleased with the manner in 
which you have treated these subjects ; the selection of topics being 
in my view judicious, and the style luminous, and well adapted to 
readers of every age, whether learned or unlearned. 

" I should rejoice to see such a school-book introduced into all 
our schools and academies. No natural science is more instructive, 
more attractive, and more practically useful, than Meteorology, 
treated as you have treated it, where the philosophical expla- 
nations of the various phenomena of the atmosphere are founded 
upon an extensive induction of facts. This science is more par- 
ticularly interesting to the young, because it explains so many 
things, that are daily occurring around them, and it thus inspires a 
taste for philosophical reasoning. I think the work cannot 
fail to be well received as a valuable addition to our list of text- 
books. 



From J. L. Comstoclc, If. D., Author of Natural Philosophy, 
Chemistry, Botany, Geology, Mineralogy, and Physiology. 

" Professor Brocklesby, of Trinity College, has submitted to my 
perusal a 'Treatise on Meteorology,' written by himself, and with 
the arrangement and science of which I am much pleased. The 
Professor wishes to have his treatise published as a school-book, 
and, considering the interest which the several subjects it em- 
braces excites in the minds of all, both old and young, rich and 
poor, I see not why such a book, when once introduced, should not 
have a large circulation. I see no reason why Meteorology, in 
many respects, has not as many claims as a school-book as Chem- 
istry or Natural Philosophy. Indeed, I should like to see Pro* 
fessor B.'s book introduced into schools as a companion of my 
Philosophy." 

Recommended also by 

JBexj. Silliman, LL. D. 
Rev. T. H. Gallaudet, 
Rev. Hoeace Hooker, 
Rev. Chas. A. Goodeich. 

This work has proved highly satisfactory in the school-room ; 
and is now the established text-book in a very large number of our 
best high schools and academies, where the natural sciences are 
taught 



20 P ro f- Brocklesby's Series. 

TIEWS OF THE MICROSCOPIC WOBLD.— ($1 12.) 

Designed for General Reading, and as a Hanoi-book for Classes in 
Natural Sciences. By Prof. Beooklesby. 

By the aid of a powerful microscope, the author has given m 
highly instructive accounts of Infusorial Animalcules, Fossil In 
fusoria, Minute Aquatic Animals, Structure. of Wood and Herbs, 
Crystallization, parts of Insects, &c, &c. 

To those who are necessarily deprived of the aid of a micro- 
scope, and even to those who have it, this is a most valuable work. 
It is clearly and pleasantly written. The sections on the Animal- 
cules, Infusoria, and Crystallizations, are very beautifully illustrated 
with large and expensive plates. The decriptions of the different 
kinds of these wonderful little animals, many of which multiply by 
millions in a few hours, are really very instructive. There i3 no 
better school library book in the world. It should be read by 
every man, woman, and child. 



PROF. BROCKLESBY'S ASTRONOMY.— ($1 25.) 
This work is printed in the first style of the art, being amply 
illustrated ; and the approval bestowed upon it by the most com- 
petent judges is such as to entitle it to the careful examination of 
teachers. 

RECOMMENDATIONS. 

I have examined it with great care, compared it with other authors 
within my reach, for the purpose of selecting a suitable manual for my 
classes pursuing the ordinary college course of mathematics. The ar- 
rangement of topics seem to be natural and scientific, and the develop- 
ment of the subject progressive. It is comprehensive and sufficient in 
scope and matter, and yet avoids the sad and frequent fault of being bare- 
ly topical and superficial, from attempting to teach too much. A special 
merit of the book is the truthfulness of its illustrations, in which are 
represented the phenomena of the heavens as they are, not as they seem, 
As a teacher I thank you for such a text book as many have been seek- 
ing in vain. 

T. W. T. Curtis, Principal High School, Hartford, Ct. 

Miss Jennette L. Douglass, of Newburgh, N. Y., who is so exten- 
sively known by the friends of education, says of Brocklesby's Astron- 
omy : " It must find its way into our best schools, as the ideas it contains 
are so clear and comprehensive, and its plate and print so plain and ele- 
gant, it contains all that is necessary for a young learner." 

Elementary works are too often mere compilations made from mate- 
rials furnished by others. This is in no sense the case in Brocklesoy's 
Astronomy. The reader will at once perceive that the author not only 
thoroughly understands his subject, but possesses the happy faculty of 
simplyfying it, and adapting it to the ordinary intelligence of the reader 



/. Olney's Geographical Series. 21 

His descriptions, aided by well drawn diagrams, make every point plain, 
and we may spfely say, we have not met with any other manual which 
so happily unfolds the elements of one of the most charming sciences 
to which human study can be directed. 

The Philadelphia Presbyterian. 

Those, who know the author will readily understand that this work is 
one of no small value. He stands among the comparatively few scientific 
men who possess strong practical characteristics, and that degree of in- 
sight and industry that enables them to comprehend and execute a text- 
book, adapted to the wants of any class of mind, 'i he book is thoroughly 
and attractively illustrated, clear and comprehensive in the text, and di- 
rect and thorough in its system of questions. 

Springfield (Mass.) Republican. 

I think Brocklesby's Astronomy the best of any with which I am ac- 
quainted. U. S. Abbott, Teacher High School, Brattleboro, Vt. 

To Professor Brocklesby, of Trinity College, Hartford, who has already 
written well upon Meteorology and Views of the Microscopic World, we 
are indebted for the Elements of Astronomy. This volume is beautifully 
and intelligibly illustrated. It brings Astronomical knowledge down to 
the present date. It is full without being diffuse, and terse without ob- 
scurity. Every scientific term is explained The rules for determining 
the distances and magnitudes of the heavenly bodies are wholly ampli- 
fied. In ad respects we can recommend the work as a very complete and 
practical elementary treatise. N. Y. Daily Times 



J. Olney's Geographical Series, 

Comprises the following Worlcs : 

Primary Geography. With Colored Maps. ... $0 25 

Quarto Geography. With several New Map3. . . . 75 

Geography and Atlas. Do. do. ... 1 12 

Outline Maps. And Key 6 00 

It is believed these works excel all others, for the following 
reasons : 

1. The clearness and correctness of definitions. 

2. The gradual arrangement of subjects. 

3. Unity of design marks the series. 

4. The nse of initial letters only. 

5. The fact that children delight m them. 

6. Their cheapness. 

The attention of teachers, whose range of subjects includes 
geography, is respectfully and particularly called to Mr. Oiney's 
Geographical Works. These works, more especially the School 
Geography and Atlas, have been in use for several years, and so far 
as the publishers have been able to ascertain, it is the general testi- 



22 J- Olnetfs Geograpnical Series. 

mony of teachers that the " Practical System of Modern Geo- 
graphy " is the best work for practical use that has ever appeared. 
But recent works have been put forth, claiming to be made upon 
superior principles, and modestly intimating that all previous stand- 
ard works are so inferior in construction as to render them de- 
servedly obsolete. Indeed it is claimed that there has been no ad- 
vance in geographical text-books for many years, until suddenly a 
new Daniel has come to judgment. In looking carefully over the 
recent inprovements so boastfully claimed, we are unable to dis- 
cover any which have not been substantially drawn from Olney's 
Geographies. 

Mr. Gluey commenced the plan of simplifying the first lesson 
and teaching a child by what is familiar to the exclusion of astron- 
omy. He commenced the plan of having only those things repre- 
sented on the maps which the pupil was required to learn. He 
originated the system of classification, and of showing the govern- 
ment, religion, &c, by symbols. He first adopted the system of 
carrying the pupil, over the earth by means of the Atlas. His 
works first contained cuts in which the dress, architecture, animals, 
internal improvements, &c, of each country are grouped, so as to 
be seen at one view. His works first contained the world as 
known to the ancients, as an aid to Ancient History, and a synopsis 
of Physical Geography with maps. In short, we have seen no 
valuable feature in any geography which has not originally ap- 
peared in these works ; and we think it not too much to claim 
that in many respects most other works are copies of these. "We 
think that a fair and candid examination will show that Olney's 
Atlas is the largest, most systematic, and complete of any yet pub- 
lished, and that the Quarto and Modern School Geographies con- 
tain more matter, and that better arranged, than any similar works. 
The attention of teachers is again called to these works, and they 
are desired to test the claims here asserted. 



TESTIMONIALS. 
From President Humphreys, D. D., Amherst College. 
Mr. J. Olney. — Dear Sir, I have examined both your improved 
School Atlas and Modern System of Geography with more than 
ordinary satisfaction. Your arrangement of topics appears to me 
better adapted to the comprehension of the child, and to follow 
more closely the order of nature, than any other elementary sys- 
tem of the kind with which I am acquainted. Instead of having 
to encounter the diagrams, problems, and definitions of Astromony 
as soon as he opens his Geography, the young learner is first pre- 
sented with the elements of the science in their simplest and most 
attractive forms. His curiosity is of course awakened. That 
which would otherwise be regarded as an irksome task, is contem- 



J. Olney^s Geographical Series. 23 

plated with pleasure. The opening mind exults in the exercise of 
its faculties, and in the ease with which it every day gathers new 
intellectual treasures. The constant use which you ohlige the 
child to make of his Atlas, I consider of a great advantage, and 
the substitution of initials for the names of countries, mountains, 
rivers, &c, a valuable improvement. There is, moreover, a con- 
densation of matter throughout, combined with a clearness and 
simplicity which cannot fail, I think, of being highly appreciated 
by all enlightened and judicious teachers. Your method of desig- 
nating the length of the principal rivers is extremely simple and 
convenient. 

From Rev. Anson W. Cummtngs, D. D., President of " Holson Conference 
College" and Ex -President of " McKendree College." 

Olney's Geography and Atlas, Revised Edition, are so beautifully 
printed, so clear and gradual in arrangement of the subjects, so correct in 
facts, so comprehensive in topics, and so cheap, that they are entitled to 
a place in every American School house and Academy. 



I have long thought Olney's Geography and Atlas a first-rate 
school-book, and the publishers of it have certainly given to it an 
attractive appearance to the teacher and pupil. I have used it, I 
think, nearly ten years of my teaching, and always found the suc- 
cessive editions reliable for accuracy, and well up to the times. 

M. F. COWDEEY, 

Supt. Schools, Sandusky. 
Similar memorials have been received from the following gen 
tlemen : 

Salem Town, LL. D. F. A. Brigham, 111. 

Pres. Lord, H. H. K P. Barrows, N. Y. 

Pres. Bates, Vt. P. Hardy, K H. 

Robert Yaux, Penn. R. S. Howard, Mass. 

M. L. Brown, K Y. E. Kingsbury, " 

M S. Hawley, Mich. E. Hall, Vt. 

J. S. Dickson, Mich. . A. K. Slade, R. I. 

N. Brittan, N. Y. J. Alwood, K Y. 

J. N. Smith, Iowa. J. Estabrook, Mich. 

T. S. Bradley, Ohio. A. D. Sturtevant, HI. 

A D. Wright, " A. G. Wilder, 

N". S. Scott, N. Y. R. C. Corey, Ark. 

Isaac Clufton, 111. . C. B. Crumb, IS". Y 
And over 500 others. 



24 The Students^ Series. 







The Students' 


Series; 








By J. S. Denman, 


A. M. 




The Students 


'? Primer, 




. , 


7 


a 


a 


Spelling-book, 


. 


• « 


13 


a 


u 


First Reader, 


» 


• • 


. 13 


a 


a 


Second " 


. 


• « 


25 


u 


a 


Third " . 




• • 


. 40 


it 


a 


Fourth " 


. 


• « 


15 


u 


a 


Fifth « 




• • 


. 94 


a 


a 


Speaker, 


• 


• , 


31 



This series of books excels all others in the following particu- 
lars : 

1. In the manner of teaching the alphabet and first principles 
of Beading, as shown in the Primer, Speller, and First Reader. 

2. In the beautiful classification and arrangement of the Speller, 
by which pupils are easily taught to spell and pronounce words 
correctly. By the aid of suffixes and prefixes, they learn to form 
derivative words, and may obtain at the same time a correct 
knowledge of their signification. 

3. The child is taught to read by beginning with words of one 
and two letters, and advancing gradually to longer words. 

4. The Primer is so arranged that each word is used in spelling 
before used in the Reading Lessons. 

5. The First Reader contains lessons of one syllable, composed 
of natural objects, such as birds, flowers, shrubs, &c, that greatly 
interest children. 

6. The same class of lessons in all the readers. 

7. The lessons are peculiarly interesting and instructive. 

8. The relation of one book to the other is very regular and 
systematic. 

9. The judicious use of plates to embellish the books and illus- 
trate the text. 

10. A judicious use of Questions, not so profuse as to embar- 
rass the pupil, nor so few as to prevent the teacher from asking 
them. 

11. The Print is very distinct and clear, from the large type in 
the Primer gradually diminishing to the common, ordinary type in 
general use. 

12. The variety of style, and the variety of subjects. 



The Students'' Series. 25 

13. The adoption of Webster's Royal Quarto Dictionary as a 
standard in spelling. 

14. In the sound moral tone pervading the whole series. No 
extracts from novels — no low and vulgar language has been al- 
lowed to blacken their pages. 

The Author was for a time the Editor of " The Student," and 
in that first produced the system on which these Headers is founded. 
It received such high encomiums that the Author thought best to 
reproduce it in book form for schools. Hence the series of Stu- 
dents' Readers. Prof. Page, late Principal of the New York State 
Normal School, said of this system, " It is the best I ever saw for 
teaching the first principles of Reading." Such testimony is of the 
highest value, and none need be afraid to use the books on such a 
recommendation. 

We present a very few names of the great number which have 
been received. 



RECOMMENDATIONS. 

The Students' Series is, in my opinion, the best in use. I believe 
a class of young students will learn twice as much, with the same 
labor, as they would from any other system. The books of this 
Series excel in the purity and attraction of their style. I have in- 
troduced them. 0. B. Cbumb, A 7 ! Y. 

I am so well pleased with them, and find them so well adapted 
to the wants of children, that I am determined to have them. 

H. H. Settee, III. 

The Students' Series I think to be far superior to any extant. 

S. O. Simo^ds, III. 

Your Students' Speller has been adopted by the School Board 
of this city. J. R. Webb, Indianapolis. 

I like your Students' books well, and shall introduce them. 

Rev. M. S. Hawley, Mich. 

I shall introduce the Students' Speller. Send me several dozen. 

O. A. Aecheb, Albany. 

I think more of the Students' Reading Books than any others 
with which I am acquainted. Ik a Satles. 

I will do all I can to introduce the Students' Series of Books, 
believing them to be the best for schools of any before the public. 

J. L. Enos, Wis. 



26 Kirkhairts Elocution. 

I am highly pleased with the Students' Series, and shall use my 
exertions to introduce it as fast as possible. 

J. Merbifield, ) j j 

Concurred in by Geo. 0. Mebbifield, J 

We use the Students' Books, and shall use no others. 

E. Lake, JST. Y. 

I use the Speller. It is just the book. 

W. M. James, Ga. 

I am using the Students' Eeaders, and like them so well that I 
urged their introduction at Monroeville, where they are now used. 

D. F. Dewolf, JVonoalJc. 

I have used the Students' Speller, and am greatly pleased with 
it. The Readers are excellent. N". B. Baekeb. 

We are using the Students' Series, and like it better than any 
other. A. Pobteb, N. Y. 

We use, and admire the Students' Series. 

J. Pooleb, IT. Y. 

I have examined the Students' Readers, and shall introduce 
them. P. J. Faeeingtojst. 

We like the Students' Books much. The Board has adopted 
'them. E. B. Coon, Covington, Ky. 

I have adopted the Students' Speller. 

Osoab Haebis, jV. J". 

We have adopted the Students' Series because we like it better 
than any other. A. L. Bingham, Mich, 

Your Students' Books are introduced here, and are well liked. 

Prof. H. Wheeler, Greencastle, Ind. 

We have adopted the Students' Speller. 

Dr. J. Nicholas, Kirtland. 

The Board of Education has adopted the Students' Speller. 
Send us five hundred. E. A. Saeldon, Syracuse. 



Kirkham's Elocution. 



This is one of the best Elocutions ever printed. It contains a 
varied and interesting selection of very useful matter, carefully ar- 
ranged. It is a standard work, and now used in some of the best 
schools in the country; among which are the Normal School, 
Philadelphia ; Lower Canada College ; Toronto Academy, &c. 



